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).

[1] https://jenkins.softwareheritage.org/job/DDOC/job/dev/1135/console

This has been broken [1] for a while.

[1] https://jenkins.softwareheritage.org/view/swh-draft/job/DDOC/job/dev/311/

Related to T3780

So it's integrated in the build.

Related to T2845

This extra requirement has been dropped from swh-storage.

Dependencies must be written in topological order, or they will be fetched
from PyPI instead of the local system

This also fixes the deposit dependency.

Related to T3172

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.

... for the time being, as swh.scanner is not published on PyPI yet

This reverts commit bdefee74.

Add swh-search and swh-icinga-plugins.

Dependencies need to be ordered in a way that ensure only development versions
will be used (not the release ones hosted on PyPI).

Related to T2188

The repository has been removed from swh-environment so no reference to it
should be kept in requirements as the pip-swh-packages script fails otherwise.

Related to T1389 T2098
Related to D2306 D2305 D2304

That's pretty big, because it includes these modules' `.git` and `.tox`,
which sums to gigabytes.

swh-web depends on swh-indexer so we must ensure swh-indexer is installed
first (otherwise the last pubished version is used instead of the one
from the sources).

and add swh-storage[listener] dep for the sake of completeness while
building the doc.

that builds the doc using source code repos located one directory above
instead of published versions of swh subpackages.

Related to T1330.