Small improvements to help the CI job tracking errors and warnings in the doc

this is a stack of 3 small revisions

1. add a hack in conf.py to make httpdomain routing table referencable

this aims at eliminating a warning generated by sphinx-build on swh-web's index, due to the presence of an explicit link to the http-routingtable.html file (which does not exists at that time, thus generated a warning).

This should be killed once

https://github.com/sphinx-contrib/httpdomain/pull/19

is accepted and released.

2. make it possible to set the SPHINXOPTS env var when building the doc with tox

This will allow the CI to run the building job with sphinx options like -W (aka "turn warnings into errors").

3. docs: prevent a 'file non included in a toctree' warning

related to T1331

Migrated from D728 (view on Phabricator)