The idea is to generalize the fact that all the swh packages should have
a README file in the root directory, but this should be included in the
sphinx documentation, for which the simplest way is to make the file
symlinked in docs/. For several reasons, it's simpler to make this
symlink creation handled by a hook/automation script at doc build time
rather than add the symlink in the git repo of the swh package.

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