[clang-tidy][docs] Rename readability check docs to Markdown [4/5] (#221640)
Tracking issue: #201242
See the [migration guide] for more information.
[migration guide]:
https://llvm.org/docs/SphinxQuickstartTemplate.html#markdown-migration-guidelines
This is the fourth part of the mechanical rename of the check
documentations in readability module.
The rewrite is provided by the next PR in this stack.