Now that sphinxcontrib-images extension has gained support for Sphinx > 2
in its 0.9.1 release, we can remove the restriction on Sphinx version in
the requirements file.

Also considering that we now rely on a virtualenv to build the doc, remove
the hacks to invocate sphinx-build and sphinx-apidoc in Makefiles.

Related to T2264

Notably remove references to old swh loader modules and exclude index.rst
files for swh-search and swh-icinga-plugins.

Related to T2188

Related to T2188

Django settings modules and swh-dataset does not contain any relevant API
documentation and apidoc generates errors and warnings while processing
them.

Related to T2188

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

Submitted-by: ardumont@

Indicate where Softwareheritage stores its data, organize by
big storage locations:

* Rocquencourt physical machines

* Rocquencourt virtual machines

* Azure-euwest

* Amazon web services

* Object storage is important and gets its own dedicated
  document

* So do the hypervisors

Also link to the existing Elasticsearch document

Reviewed-by: ardumont, vlorentz

As a visible change of this, --separate is now also used on the main docs.s.o
build. That's desirable as it makes more navigable table of contents, with a
clearly visible hierarchy of submodules---which before this change were only
visible by scrolling the entire page.

previous format %z is left unexpanded by sphinx

Currently, the dependency graph shows different nodes for, e.g., "swh.core" and
"swh.core[db]". With this change the two nodes are merged together.

While it is interesting to know which precise part of a module others depend
on, that is not the purpose of the dependency graph, which is meant to give
only a high-level view of dependencies.

Addresses T1983