[libc++][docs] Convert documentation from reST to Markdown (batch 2) (#222064)

This is the second batch of changes to migrate libc++ documentation from
rst to markdown.

I use a local pixel diff tool to compare the rendered HTML, and all
diffs seem like fixes. They tend to fall into the following categories:

- dashed bullet lists were previously not rendered, but now they are
- extra indentation interpreted as blockquotes has been removed
- single backticks are now codefont spans, not italic spans

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 #222062, which will be a standalone commit
that
renames *.rst -> *.md before this PR lands for history preservation
purposes.

GitOrigin-RevId: a0e3ae753ad39cd8f2b1b9b3e2f1ad7da3c088d2