[clang-tools-extra][docs] Convert top-level docs/*.rst to MyST Markdown (#214352)

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

[migration guide]:
https://llvm.org/docs/SphinxQuickstartTemplate.html#markdown-migration-guidelines
This is a stacked PR based on #214351 , which will be a standalone
commit that
renames *.rst -> *.md before this PR lands for history preservation
purposes.

This PR is structured as two commits:
1. Mechanical, using a fork of rst2myst
2. Agentic cleanup

This structure is used so we can determine why the markdown is the way
it is: either it's rst2myst output, or an agent thought it was a defect,
or it needed manual cleanup to build and pass validation.

I paged through all the generated HTML looking for migration artifacts,
and all of the differences I could find appear to be formatting error
corrections. Please spot check my work and approve if it looks good.

GitOrigin-RevId: dc5d098e3d4a77f89d6c4fe999bd27e0ee3eebb8