Add specific doc build option and gate on warnings

Add doc venv to align with other OpenStack projejcts (such as keystone,
heat and nova-specs) who are using 'docs' for there doc builds.  Also
enable warnerrors in pbr to raise an error if there are any docs
formatting warnings.

Change-Id: Ifa31edf3c3392106a8aa1352bdd557bde2f1b9a1
Merge "Add specific doc build option and gate on warnings"