[WIP] feat(wysiwyg): Add mdx editor

[dabbott]

Proof of concept wysiwyg editor for docs pages.

Run it locally by appending #edit=1 to any docs url.

You can see the updated mdx printed to the console.

Most things are not in the right place for production, but until we have the new app set up, I didn't want to spend time finding a better place. (I'm sure there's an existing frontmatter parser somewhere, so ignore the ai-generated one I have here)

[vercel]

@dabbott is attempting to deploy a commit to the fern Team on Vercel.

A member of the Team first needs to authorize it.