This modernises the openstack-infra documentation by switching to
openstackdocstheme. Update dependencies as required.
To remove non-relevant stuff from conf.py, I have just taken the demo
file from openstackdocstheme and lightly modified it.
It seems later sphinx has included it's own ":file:" role which now
conflicts. Change it it ":cgit_file:" in our documentation. Remove
the custom header template which no longer applies. Add the
post-2.0-pbr sphinx-based warning-as-error, which fixes the original
problem that I actually noticed that errors could slip through the
gate tests :)