[clang-tidy][docs] Rewrite modernize check docs to Markdown [2/4] (#232214)
Tracking issue: #201242
See the [migration guide] for more information.
[migration guide]:
https://llvm.org/docs/SphinxQuickstartTemplate.html#markdown-migration-guidelines
This is the second part of rewriting check documentations in modernize
module from reST to MyST Markdown.
AI Usage: This was prepared with rnk's fork of rst2myst and
GPT5.6-assisted (both Codex and AmpCode) cleanup.
I manually verified that the documentation renders as expected.
Preview site:
https://broken.life/llvm-staging/modernize-markdown-port/index.html