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

docs/_templates/footer.html

@@ -4,6 +4,8 @@
 <!-- Empty para to force a blank line after "Built with Sphinx ..." -->
 <p></p>
 
+<p>This documentation is for QEMU version {{ version }}.</p>
+
 {% trans path=pathto('about/license') %}
 <p><a href="{{ path }}">QEMU and this manual are released under the
 GNU General Public License, version 2.</a></p>