[clang-tidy][docs] Rename check docs in six modules to Markdown (#217671)
Tracking issue: #201242
See the [migration guide] for more information.
[migration guide]:
https://llvm.org/docs/SphinxQuickstartTemplate.html#markdown-migration-guidelines
This is the mechanical rename of the check documentations in six
modules.
The rewrite is provided by the next PR in this stack.