Docs: Fix broken links ending with ".mdx"

When I checked "How to contribute" page on storybook.js.org, a link
titled "Continue reading our contributor covenant" pointed to GitHub's
404 page. The last item of the URL path was "CODE_OF_CONDUCT.mdx", which
is not common filename people uses, therefore I suspected this is not
the only broken link in the docs.

And I was right. I ran the below command (rg = RipGrep) then found
several 404 links.

```
$ cd docs
$ rg "https?:[\S)]+mdx\)" --sort path
```

Git blame told me that some of ".md" to ".mdx" migration works
accidentally changed non-docs URLs ending with ".md" too.

StackBlitz URL in docs/api/main-config/main-config-indexers.mdx is not
actually a dead link, as the service redirects to the `README.md`.
However, the redirection takes loooong so I replaced that as well.

There are still two links that match to the above regexp:

* <https://github.com/storybookjs/storybook/blob/next/docs/get-started/setup.mdx>
  at docs/contribute/documentation/new-snippets.mdx L42
* <https://marketplace.visualstudio.com/items?itemName=unifiedjs.vscode-mdx>
  at docs/writing-docs/mdx.mdx

but those two are both valid (alive) URLs.

docs/api/main-config/main-config-indexers.mdx

@@ -341,7 +341,7 @@ Some example usages of custom indexers include:
 <details>
   <summary>Generating stories with an alternative API</summary>
 
-  You can use a custom indexer and builder plugin to create your API to define stories extending the CSF format. To learn more, see the following [proof of concept](https://stackblitz.com/edit/github-h2rgfk?file=README.mdx) to set up a custom indexer to generate stories dynamically. It contains everything needed to support such a feature, including the indexer, a Vite plugin, and a Webpack loader.
+  You can use a custom indexer and builder plugin to create your API to define stories extending the CSF format. To learn more, see the following [proof of concept](https://stackblitz.com/edit/github-h2rgfk?file=README.md) to set up a custom indexer to generate stories dynamically. It contains everything needed to support such a feature, including the indexer, a Vite plugin, and a Webpack loader.
 </details>
 
 <details>

docs/configure/upgrading.mdx

@@ -31,7 +31,7 @@ After running the command, the script will:
 * Run the relevant [automigrations](../migration-guide/index.mdx#automatic-upgrade) factoring in the [breaking changes](../migration-guide/index.mdx#major-breaking-changes) between your current version and the specified version
 
 <Callout variant="info">
-  In addition to running the command, we also recommend checking the [MIGRATION.md file](https://github.com/storybookjs/storybook/blob/next/MIGRATION.mdx), for the detailed log of relevant changes and deprecations that might affect your upgrade.
+  In addition to running the command, we also recommend checking the [MIGRATION.md file](https://github.com/storybookjs/storybook/blob/next/MIGRATION.md), for the detailed log of relevant changes and deprecations that might affect your upgrade.
 </Callout>
 
 ### Verifying the upgrade

docs/contribute/framework.mdx

@@ -27,7 +27,7 @@ The library or libraries your framework supports may have different major versio
 
 ### 3. Write the documentation
 
-Before writing any code, write a helpful README that contains installation instructions and a list of available features. Use the [README for `@storybook/nextjs`](https://github.com/storybookjs/storybook/blob/next/code/frameworks/nextjs/README.mdx) as a template. Writing the documentation first helps guide your other work.
+Before writing any code, write a helpful README that contains installation instructions and a list of available features. Use the [README for `@storybook/nextjs`](https://github.com/storybookjs/storybook/blob/next/code/frameworks/nextjs/README.md) as a template. Writing the documentation first helps guide your other work.
 
 ### 4. Author the framework itself
 

docs/contribute/index.mdx

@@ -10,7 +10,7 @@ Storybook is a community-oriented open source project that welcomes contribution
 
 ## Contributor covenant
 
-In the interest of fostering an open and welcoming environment, we as contributors and maintainers pledge to making participation in our project and our community a harassment-free experience for everyone, regardless of age, body size, disability, ethnicity, gender identity and expression, level of experience, nationality, personal appearance, race, religion, or sexual identity and orientation. [Continue reading our contributor covenant »](https://github.com/storybookjs/storybook/blob/next/CODE_OF_CONDUCT.mdx)
+In the interest of fostering an open and welcoming environment, we as contributors and maintainers pledge to making participation in our project and our community a harassment-free experience for everyone, regardless of age, body size, disability, ethnicity, gender identity and expression, level of experience, nationality, personal appearance, race, religion, or sexual identity and orientation. [Continue reading our contributor covenant »](https://github.com/storybookjs/storybook/blob/next/CODE_OF_CONDUCT.md)
 
 ## Ways to contribute
 

docs/faq.mdx

@@ -195,7 +195,7 @@ We're actively working to provide a better way to address this situation, but in
 
 ## Is it possible to browse the documentation for past versions of Storybook?
 
-With the release of version 6.0, we updated our documentation as well. That doesn't mean that the old documentation was removed. We kept it to help you with your Storybook migration process. Use the content from the table below in conjunction with our [migration guide](https://github.com/storybookjs/storybook/blob/next/MIGRATION.mdx).
+With the release of version 6.0, we updated our documentation as well. That doesn't mean that the old documentation was removed. We kept it to help you with your Storybook migration process. Use the content from the table below in conjunction with our [migration guide](https://github.com/storybookjs/storybook/blob/next/MIGRATION.md).
 
 We're only covering versions 5.3 and 5.0 as they were important milestones for Storybook. If you want to go back in time a little more, you'll have to check the specific release in the monorepo.
 
@@ -246,7 +246,7 @@ We're only covering versions 5.3 and 5.0 as they were important milestones for S
 |                  | Environment variables                           | [See current documentation](./configure/environment-variables.mdx)                                   | [See versioned documentation](https://github.com/storybookjs/storybook/tree/release/5.3/docs/src/pages/configurations/env-vars)                                                                                                                                      | [See versioned documentation](https://github.com/storybookjs/storybook/tree/release/5.0/docs/src/pages/configurations/env-vars)                          |
 | Builders         | Introduction                                    | [See current documentation](./builders/index.mdx)                                                    | Non existing feature or undocumented                                                                                                                                                                                                                                 | Non existing feature or undocumented                                                                                                                     |
 |                  | Vite                                            | [See current documentation](./builders/vite.mdx)                                                     | Non existing feature or undocumented                                                                                                                                                                                                                                 | Non existing feature or undocumented                                                                                                                     |
-|                  | Webpack                                         | [See current documentation](./builders/webpack.mdx)                                                  | [See versioned documentation](https://github.com/storybookjs/storybook/tree/release/5.3/docs/src/pages/configurations/custom-webpack-config/index.mdx)                                                                                                               | [See versioned documentation](https://github.com/storybookjs/storybook/tree/release/5.0/docs/src/pages/configurations/custom-webpack-config/index.mdx)   |
+|                  | Webpack                                         | [See current documentation](./builders/webpack.mdx)                                                  | [See versioned documentation](https://github.com/storybookjs/storybook/tree/release/5.3/docs/src/pages/configurations/custom-webpack-config/index.md)                                                                                                               | [See versioned documentation](https://github.com/storybookjs/storybook/tree/release/5.0/docs/src/pages/configurations/custom-webpack-config/index.md)   |
 |                  | Builder API                                     | [See current documentation](./builders/builder-api.mdx)                                              | Non existing feature or undocumented                                                                                                                                                                                                                                 | Non existing feature or undocumented                                                                                                                     |
 | Addons           | Introduction                                    | [See current documentation](./addons/index.mdx)                                                      | [See versioned documentation](https://github.com/storybookjs/storybook/tree/release/5.3/docs/src/pages/addons/writing-addons)                                                                                                                                        | [See versioned documentation](https://github.com/storybookjs/storybook/tree/release/5.0/docs/src/pages/addons/writing-addons)                            |
 |                  | Install addons                                  | [See current documentation](./addons/install-addons.mdx)                                             | [See versioned documentation](https://github.com/storybookjs/storybook/tree/release/5.3/docs/src/pages/addons/using-addons/)                                                                                                                                         | [See versioned documentation](https://github.com/storybookjs/storybook/tree/release/5.0/docs/src/pages/addons/using-addons/)                             |
@@ -301,7 +301,7 @@ We're only covering versions 5.3 and 5.0 as they were important milestones for S
 |                  | CLI options                                     | [See current documentation](./api/cli-options.mdx)                                                   | [See versioned documentation](https://github.com/storybookjs/storybook/tree/release/5.3/docs/src/pages/configurations/cli-options)                                                                                                                                   | [See versioned documentation](https://github.com/storybookjs/storybook/tree/release/5.0/docs/src/pages/configurations/cli-options)                       |
 
 <Callout variant="info">
-  If you have stories written with the older `storiesOf` format, it was removed in Storybook 8.0 and is no longer maintained. We recommend that you migrate your stories to CSF. See the [migration guide](./migration-guide/index.mdx#storiesof-to-csf) for more information. However, if you need, you can still access the old `storiesOf` [documentation](https://github.com/storybookjs/storybook/blob/release/5.3/docs/src/pages/formats/storiesof-api/index.mdx) for reference.
+  If you have stories written with the older `storiesOf` format, it was removed in Storybook 8.0 and is no longer maintained. We recommend that you migrate your stories to CSF. See the [migration guide](./migration-guide/index.mdx#storiesof-to-csf) for more information. However, if you need, you can still access the old `storiesOf` [documentation](https://github.com/storybookjs/storybook/blob/release/5.3/docs/src/pages/formats/storiesof-api/index.md) for reference.
 </Callout>
 
 ## What icons are available for my toolbar or my addon?

docs/migration-guide/from-older-version.mdx

@@ -119,7 +119,7 @@ The automatic upgrade should get your Storybook into a working state. If you enc
 If you prefer to debug yourself, here are a few useful things you can do to help narrow down the problem:
 
 1. Try removing all addons that are not in the `@storybook` npm namespace (make sure you don't remove the `storybook` package). Community addons that work well with 7.x might not yet be compatible with 8.0, and this is the fastest way to isolate that possibility. If you find an addon that needs to be upgraded to work with Storybook 8, please post an issue on the addon’s repository, or better yet, a pull request to upgrade it!
-2. Another debugging technique is to bisect to older prerelease versions of Storybook to figure out which release broke your Storybook. For example, assuming that the current prerelease of Storybook is `8.0.0-beta.56`, you could set the version to `8.0.0-alpha.0` in your `package.json` and reinstall to verify that it still works (`alpha.0` should be nearly identical to `7.6.x`). If it works, you could then try `8.0.0-beta.0`, then `8.0.0-beta.28` and so forth. Once you’ve isolated the bad release, read through its [CHANGELOG](https://github.com/storybookjs/storybook/blob/next/CHANGELOG.mdx) entry and perhaps there’s a change that jumps out as the culprit. If you find the problem, please submit an issue or pull request to the Storybook monorepo and we’ll do our best to take care of it quickly.
+2. Another debugging technique is to bisect to older prerelease versions of Storybook to figure out which release broke your Storybook. For example, assuming that the current prerelease of Storybook is `8.0.0-beta.56`, you could set the version to `8.0.0-alpha.0` in your `package.json` and reinstall to verify that it still works (`alpha.0` should be nearly identical to `7.6.x`). If it works, you could then try `8.0.0-beta.0`, then `8.0.0-beta.28` and so forth. Once you’ve isolated the bad release, read through its [CHANGELOG](https://github.com/storybookjs/storybook/blob/next/CHANGELOG.md) entry and perhaps there’s a change that jumps out as the culprit. If you find the problem, please submit an issue or pull request to the Storybook monorepo and we’ll do our best to take care of it quickly.
 
 ## Optional migrations
 

docs/migration-guide/index.mdx

@@ -133,7 +133,7 @@ The automatic upgrade should get your Storybook into a working state. If you enc
 If you prefer to debug yourself, here are a few useful things you can do to help narrow down the problem:
 
 1. Try removing all addons that are not in the `@storybook` npm namespace (make sure you don't remove the `storybook` package). Community addons that work well with 7.x might not yet be compatible with 8.0, and this is the fastest way to isolate that possibility. If you find an addon that needs to be upgraded to work with Storybook 8, please post an issue on the addon’s repository, or better yet, a pull request to upgrade it!
-2. Another debugging technique is to bisect to older prerelease versions of Storybook to figure out which release broke your Storybook. For example, assuming that the current prerelease of Storybook is `8.0.0-beta.56`, you could set the version to `8.0.0-alpha.0` in your `package.json` and reinstall to verify that it still works (`alpha.0` should be nearly identical to `7.6.x`). If it works, you could then try `8.0.0-beta.0`, then `8.0.0-beta.28` and so forth. Once you’ve isolated the bad release, read through its [CHANGELOG](https://github.com/storybookjs/storybook/blob/next/CHANGELOG.mdx) entry and perhaps there’s a change that jumps out as the culprit. If you find the problem, please submit an issue or pull request to the Storybook monorepo and we’ll do our best to take care of it quickly.
+2. Another debugging technique is to bisect to older prerelease versions of Storybook to figure out which release broke your Storybook. For example, assuming that the current prerelease of Storybook is `8.0.0-beta.56`, you could set the version to `8.0.0-alpha.0` in your `package.json` and reinstall to verify that it still works (`alpha.0` should be nearly identical to `7.6.x`). If it works, you could then try `8.0.0-beta.0`, then `8.0.0-beta.28` and so forth. Once you’ve isolated the bad release, read through its [CHANGELOG](https://github.com/storybookjs/storybook/blob/next/CHANGELOG.md) entry and perhaps there’s a change that jumps out as the culprit. If you find the problem, please submit an issue or pull request to the Storybook monorepo and we’ll do our best to take care of it quickly.
 
 ## Package structure changes