[tox] Update doc testenv to account for documentation restructuring * Build doc from `doc` directory. * Use `sphinx-build` directly instead of `make` as this now works and allows to build within .tox temporary environment (instead of doc/_build), preferable for a "test" action. Related to #5447161.

version=3
opts=uversionmangle=s/(rc|a|b|c)/~$1/ \
http://pypi.debian.net/cubicweb/cubicweb-(.+)\.(?:zip|tgz|tbz|txz|(?:tar\.(?:gz|bz2|xz)))