Switch recommonmark to myst-parser

[ChristianKuehnel]

Just saw on Discourse that the Markdown addon "recommonmark" for Sphinx will be deprecated. This is used in generating our llvm.org website. https://llvm.discourse.group/t/switch-recommonmark-to-myst-parser/3259

The recommended alternative is MyST-Parser. So I guess it would make sense for LLVM to also switch at some point in time...

Do we consider this in scope for our working group?

I suppose this would mean changing the Sphinx config in the repo and upgrading all machines building the documentation.

[polyzen]

Implemented in https://github.com/llvm/llvm-project/commit/b7ff03206d668cd5a620a9d4e1b22ea112ed56e3