docs: Add license note to the HTML page footer

docs: Add license note to the HTML page footer

The standard Sphinx/RTD HTML page footer gives a copyright line
(based on the 'copyright' variable set in conf.py) and a line "Built
with Sphinx using a theme provided by Read the Docs" (which can be
disabled via the html_show_sphinx variable, but we leave it enabled).
As a free software project, we'd like to also mention the license
QEMU and its manual are released under.

Add a template footer.html which defines the 'extrafooter' block that
the RtD theme provides for this purpose.  The new line of text will
go below the existing copyright and sphinx-acknowledgement lines.
(Unfortunately the RTD footer template does not permit putting it
after the copyright but before the sphinx-acknowledgement.)

We use the templating functionality to make the new text also be a
hyperlink to the about/license.html page of the manual.

Unlike rst files, HTML template files are not reported to our depfile
plugin, so we maintain a manual list in meson.build.  New template
files should be rare, so not being able to auto-generate the
dependency info is not too awkward.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20210705095547.15790-7-peter.maydell@linaro.org