docs: fix handling of static assets in build dir

docs: fix handling of static assets in build dir

We previously added a hack to symlink CSS files from the source dir into
the build dir, to allow the website to be browsed locally. We should
have also done this for any images.

This change merges several variables into one "$(assets)" so that we
treat all static files in the root dir the same way.

Reviewed-by: Laine Stump <laine@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>