[libc++][docs] Rename documentation files to Markdown (batch 2) (#222062) Tracking issue: #201242 See the [migration guide] for more information. [migration guide]: https://llvm.org/docs/SphinxQuickstartTemplate.html#markdown-migration-guidelines This is the initial straight rename commit. It will probably break the docs build, but it has to be a separate PR for blame preservation purposes. GitOrigin-RevId: b08d731d282fc5d9ddcd08c66a135af1dee6ad13