[OpenMP][docs] Rename reStructuredText files to Markdown (#222191)

Tracking issue: #201242
See the [migration guide] for more information.

[migration guide]:
https://llvm.org/docs/SphinxQuickstartTemplate.html#markdown-migration-guidelines

This is the initial straight rename commit. It will probably break the
docs build, but it has to be a separate PR for blame preservation
purposes.

Validation:

- Confirmed this commit contains exactly 34 `.rst` to `.md` renames and
no content changes.
- The complete stacked migration is validated in the follow-up PR.

GitOrigin-RevId: 73b9d5341de05b0b433c1c133104ab1b74ab6e13