Finish the transition to the openstackdocs theme in accord
with the OpenStack manuals project migration spec [1].
Also remove the already broken autodoc build since
it generates warnings (which we treat as errors) with
the new theme setup.
[1] https://specs.openstack.org/openstack/docs-specs/specs/pike/os-manuals-migration.html
Partial-Bug: #1706181
Needed-By: Ieea33262101a1d2459492c1c8aaac5fe042279f6
Depends-On: Ia750cb049c0f53a234ea70ce1f2bbbb7a2aa9454
Change-Id: Ifc80fc56648cef74c85464321d1850e8c68449a0
8da4333c7f