Add markdown support for .mdx files

[ctmayn]

Implements https://github.com/github/repos/issues/8873

Adds basic support for rendering .mdx files as markdown. Non-markdown syntax will just be printed out as plain text

[AdamShwert]

LGTM

[wooorm]

Hi! MDX person here. Looks like this is now alive on the website.

I’m not sure this is a good idea. There are some MDX files that are indeed close to markdown. But this also looks really bad for other files. Some examples:

• https://github.com/remcohaszing/remcohaszing.github.io/blob/main/pages/index.mdx
• https://github.com/mdx-js/mdx/blob/main/docs/index.mdx

MDX-the-language has a benefit for programmers that want to use their own React/Vue/etc components.
MDX users can also use .md files with plain markdown if they don’t need to program or use components and want to be compatible with GH.
I believe that seeing code is fine for programmers.

The “Non-markdown syntax will just be printed out as plain text” is not correct, as can be seen.

What is the reasoning in 8873? Can I convince the people in that discussion that this may not be an improvement?