[docs] Rename selected libc docs to Markdown (#208354)
Tracking issue: https://github.com/llvm/llvm-project/issues/201242
Migration guide docs:
https://llvm.org/docs/SphinxQuickstartTemplate.html#markdown-migration-guidelines
RFC:
https://discourse.llvm.org/t/rfc-make-myst-markdown-the-llvm-docs-format-rip-rest/90840
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.
