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

[Tobias Hieta via cfe-commits]

tru wrote:

Instead of updating the module manually on the bot, I still think it's a good idea to add a requirements.txt that should be installed before running the build, ideally in a virtualenv. WDYT @andreil99 

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