Ian Wienand 882b730fdf Update to openstackdocstheme

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 :)

Change-Id: Ic7bec57b971bb4c75fc839e7269d1f69a576b85c

2018-06-25 11:19:43 +10:00