Follow the PTI for docs

Use sphinx-build instead of the pbr sphinx extention for building docs
as instructed by the PTI[1].

It fixes the header formatting for the index page, as the headers weren't
rendering at all.

[1] https://governance.openstack.org/tc/reference/pti/python.html

Change-Id: Ibac2b45ecfab4a7e575d097ecb9fc2c5e57b81cf
2 files changed