docs: Add QEMU version information to HTML footer

docs: Add QEMU version information to HTML footer

Add a line to the HTML document footer mentioning the QEMU version.
The version information is already provided in very faint text below
the QEMU logo in the sidebar, but that is rather inconspicious, so
repeating it in the footer seems useful.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20210705095547.15790-8-peter.maydell@linaro.org