[docs] Rename selected clang docs to Markdown (#206181)
Tracking issue: https://github.com/llvm/llvm-project/issues/201242
Migration guide docs:
https://llvm.org/docs/SphinxQuickstartTemplate.html#markdown-migration-guidelines
RFC:
https://discourse.llvm.org/t/rfc-make-myst-markdown-the-llvm-docs-format-rip-rest/90840
Rename 20 selected clang docs from reStructuredText to MyST Markdown
filenames.
This PR is intentionally limited to the rename phase and required
reference updates. The mechanical conversion and follow-up MyST cleanup
are in the stacked PR.
