[clang-tidy][docs] Rename bugprone check docs to Markdown [2/4] (#214418)

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 for part 2/4 of the remaining bugprone
check documentation. The rewrite is provided by the next PR in this
stack.

GitOrigin-RevId: 997fd9c559bb8584fdf437b28404f9d2565c3b8e
20 files changed