docs: ensure HTML/images are built before running reference tests

The 'check-html-references' test will process the built HTML files,
so they must exist before it is run, along with any images that
they point to.

If using the older 'configure_file' command, no changes are needed
since that always gets executed at 'meson setup' time, rather than
at 'meson compile' time.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>