These changes allow to build all SWH documentations (developmment, system
administration and user) using a single sphinx-build command, meaning they
are now merged into a single one with shared index of references.

Development documentation is now rooted to docs/devel, user one to docs/user
and sysadmin one to docs/sysadm so a good amount of files were moved.

A couple of configuration files and makefiles were updated to reflect that change
and the building / cleaning processes were made more reliable.

Calling make in root directory of swh-docs will execute the development build
of the documentation in a tox environment while calling make in docs folder
will use the current virtualenv.

It remains possible to build each documentation in a standalone way by
calling make in their root directory.

Closes #4496

This nests the historical roadmap under a specific roadmap/index
toctree, so that they don't all show up in the sidebar.