[docs] Rename selected PDB, DirectX, and GlobalISel docs to Markdown (#217157)
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.