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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Ah, thank you for the extra information! We definitely cannot go back to recommonmark, but perhaps @cor3ntin can find a way to restore the older behavior there. However, I think finding the minimal set of dependencies might be brittle in terms of long-term maintenance (so if it turns out it's significant effort to support the old behavior, I think the extra package may be the price we pay for getting off an unmaintained package).

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