Addon-docs: MDX support

[shilman]

Issue: #7101

What I did

Note this builds on #7119 and is temporarily targeted at that PR to highlight the diffs. Should be updated after that gets merged.

• Add an MDX loader for storybook that converts MDX files into module story format
• Add presets for addon-docs
• Add MDX examples

How to test

To test the MDX loader:

yarn jest --testPathPattern mdx-compiler-plugin.test.js

To test end-to-end:

cd examples/official-storybook
yarn storybook
# Check out stories under Addons|Docs/{stories,mdx}

[vercel]

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[tmeasday]

I don't really have enough expertise to say if the AST code is good or not but this seems pretty nice