Add support for Sphinx Docs.

Also turn on doc API autoindexing.

This depends on https://review.openstack.org/30520 landing in pbr and a
new point release being cut, as it uncovered a bug.

Change-Id: I8f4ffc4f7c54c7b207d6d52e74ccab020596e602
Merge "Add support for Sphinx Docs."