[docs] Rename misc rst docs to md (#210208)

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.

Many of these miscellaneous .rst files are not actually part of a real
docs build, since docs/conf.py does not exist for many of them.

GitOrigin-RevId: c42ce32264f94a8800f4250d7b716cca55f77df6