doc: rebuild with meson whenever a file changes

doc: rebuild with meson whenever a file changes

Add proper support for calling sphinx whenever a file in the doc
directory changes. This is accomplished by using a wrapper script
for sphinx, which runs sphinx but also emits a gcc-format dependency
file listing all the doc files. This is used by ninja so that any
change to the doc files triggers a rebuild of the docs.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Aaron Conole <aconole@redhat.com>
Acked-by: Luca Boccassi <bluca@debian.org>