gitlab-ci: Enable docs in the centos job

gitlab-ci: Enable docs in the centos job

We just ran into a problem that the docs don't build on RHEL8 / CentOS 8
anymore. Seems like these distros are using one of the oldest Sphinx
versions that we still have to support. Thus enable the docs build in
the CI on CentOS so that such bugs don't slip in so easily again.

Message-Id: <20220104091240.160867-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>