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 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 fixes an import error during sphinx-apidoc processing.

This extra requirement has been dropped from swh-storage.

This also fixes the deposit dependency.

Related to T3172

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

This reverts commit bdefee74.

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

It's no longer in swh-environment as the repository is archived now

This avoids problem as mentioned in T1734#32762

Remove the depth-one tree in py-deps-swh.svg doc as the root.

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

- back to assets only building in each package
- run sphinx-autodoc on the whole swh package at once

we do not really need a local standalone buildable sphinx project in each swh
subpackage, so running a full 'make html' in each of them is not required any
more.

Since we want to build the doc at once instead of building each swh subpackage
independantly, we need to run apidoc on the whole swh package.

This is done by actually running the apidoc on the pip installed swh
packages instead of the source repositories, since sphinx-apidoc does
not seem to support multiple MODULE_PATH as cmdline argument.

So it's best to use tox to build the doc.

Related to T1330.