Update Markdown-related meta-docs with info about how to use Markdown linters

[teoli2003]

With more and more docs translated into Markdown (#7853), writers are starting to use Markdown tools and especially linters.

For example, I've installed markdownlint in my editor so that I'm aware of errors and write better-formatted docs.

I noticed that the default configuration is asking me to change mark-up we like to mark-up we don't like. E.g., it prefers plain pre blocks with that format:

    Pre block content

when we prefer:

 ```
 Pre block content
 ```

We should list these things, and maybe give info about how to configure (very) common markdown linter.

To solve this, we should:

1. Have a list of common tools
2. Have a list of correct Markdown structures we want to avoid.
3. Update the meta-docs themselves.

Note that if we want to enforce 2. in the future, we will still need this info :-)

Feel free to add info about 1. and 2. here so we can start collecting this info.

[teoli2003]

Mardownlint is now run automatically with Husky, and as a test in PRs. The configuration file is also stored in GitHub. This issue is no more useful.