[clang-tools-extra] [clang-tidy][docs] Rename short check docs to Markdown (PR #210466)

[Baranov Victor via cfe-commits]

https://github.com/vbvictor approved this pull request.

Do I understand correctly that transition of long check docs to Markdown will be up for review soon as well?

https://github.com/llvm/llvm-project/pull/210466