Now that all sphinx warnings have been fixed by @vlorentz, it would
be great to avoid introducing new ones as much as possible.
This diff showcases how a `sphinx` environment can be added to `tox`
to build and check the documentation proper to a swh package.
As some external references could not be resolved in a single package
scope, related sphinx warnings are suppressed for the build to succeed
but other ones are not.
This would allow to catch any rst errors prior landing a diff.
Ideally a new parallel step in our Jenkins pipelines to build and
check the doc should be added. We already have a CI docker image
containing all the required tools to build the doc so this should
not be too complicated to integrate.