[clang-doc] Add a Mustache Markdown generator (#177221)
Adds a Markdown generator that uses Mustache templates. This patch adds
the templates themselves and implements changes to the JSONGenerator to
allow for the creation of specific files needed by the MD tests like
`all-files.json`.
This backend should be considered experimental. It satisfies all the
same tests that the current MD backend is tested against, but those
don't seem to provide full coverage for all functionality inside that
backend. It also doesn't output everything provided by JSON. It doesn't
use the MD unittests because the Mustache templates must currently be
written to files.
