[clang] [flang] [llvm] [Documentation] Replace recommonmark by myst-parser (PR #65664)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> I've adjusted the Gentoo packages, and can live with it. I just find it sad that even though there was an explicit comment on why the `ImportError` fallback was added, it was just removed without a second thought or even an attempt to communicate with me (given `git blame` would have attributed the original change to me).

I think it was simply overlooked by accident; nobody "owns" this functionality in community and so when we needed to get off the archived package, we took the offered patch which was based on advice from the recommonmark maintainers on how to port to myst-parser. Given that there's no post-commit bot testing this configuration, it's easy enough for this kind of thing to slip through the cracks. FWIW, if we do end up restoring the old behavior to help Gentoo folks out, I think we need some sort of test coverage for it and we'd appreciate your help in getting that set up so we don't regress again (assuming we're able to restore the old behavior).

https://github.com/llvm/llvm-project/pull/65664