[libcxx-commits] [libcxx] [libc++][docs] Rename remaining rst docs to md [1/2] (PR #233994)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 requested changes to this pull request.

The `ReleaseNotes/` were left out specifically because they can easily be replaced by just updating the ReleaseNotesTemplate and waiting a few releases.

https://github.com/llvm/llvm-project/pull/233994