Convert qemu-ga-ref to rST format. This includes dropping
the plain-text, pdf and info format outputs for this document;
as with all our other Sphinx-based documentation, we provide
HTML and manpage only.

The qemu-ga-ref.rst is somewhat more stripped down than
the .texi was, because we do not (currently) attempt to
generate indexes for the commands, events and data types
being documented.

As the GA ref is now part of the Sphinx 'interop' manual,
we can delete the direct link from index.html.in.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200925162316.21205-9-peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Unicode legacy literal dumbed down to plain string literal, TODO
comment on displaying QEMU version added]
Signed-off-by: Markus Armbruster <armbru@redhat.com>