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

[Michał Górny via llvm-commits]

mgorny 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).

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