[clang-tools-extra][docs] Rename top-level docs/*.rst to *.md (#214351)

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: ab9aaf5ea6d0c92d282be5bdf0676cb58c1f252f
19 files changed