[lldb] Enable support for Markdown documentation pages (#89716)

RST is powerful but usually too powerful for 90% of what we need it for.
Markdown is easier to edit and can be previewed easily without building
the entire website.

This copies what llvm does already, making myst_parser optional if you
only want man pages.

Previously we had Markdown enabled in
8b95bd33 but that got reverted. That did
this in a different way but I've gone with the standard llvm set this
time.

I intend the first Markdown pages to be the remote protocol extension
docs, as they are not in any set format right now.