It is a dependency required to build psycopg-c that does not get installed
in the virtualenv used to build the documentation as we use the pip option
--no-build-isolation that skips installation of build dependencies specified
by PEP 518.

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

With recent enough versions of setuptools/setuptools-scm, the pip
dependency resolver is now capable of sorting out our full dependency
tree. We can just install all packages at once (which does the
dependency resolution step once).

To speed up the process even more, inline the few "build" (setup.py)
dependencies to allow disabling build isolation (which creates a new
venv and installs setuptools-scm in it for every single package).

It's needed by the ci to be able to reference the api links

Related to T3307

Installation is implicit when using tox but not in a custom make script.

The new pip resolver ends up backtracking numerous packages when
installing all depencies in a single pip command but this is
terribly slow and long to execute.

A workaround is to install each swh package one at a time by
executing pip multiple times, no backtracking happens in that
case and it reduces considerably the overall installation time.

Another advantage of proceeding like this is that it ensures each
swh package will be installed in development mode.

Also increase tox verbosity when calling make to have some feeback
of what it is doing.