This reverts commit 457c9f64.

This commit removed both the source copy step and the actual linking of the docs
sources. Turns out that the latter is "somewhat" necessary for a proper doc
build.

Close T2386

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.

Related to T2367

Related to T2367

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

This reverts commit bdefee74.

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.