[clang-tidy][docs] Rename bugprone check docs to Markdown [4/4] (#214426)

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

GitOrigin-RevId: b272bfe2177312f07c3d0ca08e80cdb5a1d6bdff
22 files changed