Even though [pbr] warnerrors = true is specified,
somehow sphinx warnings are not treated as errors.
If we use sphinx-build, -W option detects document warnings
and it leads to better documentation quality.
Fix a warning in the existing document too.
Change-Id: Icbb39686c7a24f0040d998fb14e14c63613da190
In addition, to remove the contrib/ way to load commands since
it is not implemented at all.
Change-Id: Id15eb10e7cb086d55d2bcad4f719f9120c6f7d1a
Closes-Bug:#1486824
