Refactor the sphinx-dev build environment to get rid of the link-stamp step

by installing swh packages in standard (non-editable) mode, so we have an
easy to specify swh/ directory in which all the swh packages are installed.

The main drawback of this solution is we loose the possibility of rebuilding
the doc (in tox) while editing a package's documentation without rebuilding
the tox env (i.e. using tox -r). But IMHO this usage has never been
adpoted by anyone, so it's much simpler that way.