Add .mdx extension support

[davidknezic]

Q	A
Bug fix?	no
New feature?	yes
Breaking changes?	no
Deprecations?	no
New tests added?	not needed
Fixed tickets	Fixes #2109
License	MIT

Description

Make it possible to open .mdx files which are Markdown files with the possibility of adding JavaScript (JSX) code. MDX is already very widespread and is still gaining a lot of popularity.

The main effect this change has is that it displays files with the .mdx ending in the sidebar and allows them to be opened.

As the files are basically Markdown files, they are already displayed correctly. Any JSX inside of the files also already looks good and doesn't need any special treatment:

Saving works perfectly fine.

This PR conflicts with and closes #3424 that was opened recently. It adds the MDX extension to a couple more places in the source code / docs.

[pythonicode]

Would be nice to get a reviewer here. This is pretty important for developers using React and MDX

[AdrianoCahete]

@Jocs the latest build has this feature?

[Jocs]

@AdrianoCahete I have not release the new version with this feature yet.

[kemo-1]

I am currently using https://markdoc.dev/ its trying to achive the same effect as mdx but with much better performance and flexability

basic example:

This is shown no matter what.

{% if $myFunVar %}
Only appear if $myFunVar!
{% /if %}

also you can do what mdx dose but with better experince for authors and devolopers by seperating your code from your content while also providing small components that the authors can use

[fazi-gondal]

Please make support for mdx file