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

MAINTAINERS

@@ -3451,6 +3451,7 @@ S: Maintained
 F: docs/conf.py
 F: docs/*/conf.py
 F: docs/sphinx/
+F: docs/_templates/
 
 Miscellaneous
 -------------

docs/_templates/footer.html

@@ -0,0 +1,12 @@
+{% extends "!footer.html" %}
+{% block extrafooter %}
+
+<!-- Empty para to force a blank line after "Built with Sphinx ..." -->
+<p></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>
+{% endtrans %}
+{{ super() }}
+{% endblock %}

docs/meson.build

@@ -44,6 +44,7 @@ if build_docs
                           meson.source_root() / 'docs/sphinx/qapidoc.py',
                           meson.source_root() / 'docs/sphinx/qmp_lexer.py',
                           qapi_gen_depends ]
+  sphinx_template_files = [ meson.source_root() / 'docs/_templates/footer.html' ]
 
   have_ga = have_tools and config_host.has_key('CONFIG_GUEST_AGENT')
 
@@ -76,7 +77,7 @@ if build_docs
                 output: 'docs.stamp',
                 input: files('conf.py'),
                 depfile: 'docs.d',
-                depend_files: sphinx_extn_depends,
+                depend_files: [ sphinx_extn_depends, sphinx_template_files ],
                 command: [SPHINX_ARGS, '-Ddepfile=@DEPFILE@',
                           '-Ddepfile_stamp=@OUTPUT0@',
                           '-b', 'html', '-d', private_dir,