this aims at eliminating a warning generated by sphinx-build on swh-web's
index, due to the presence of an explicit link to the http-routingtable.html
file (which does not exists at that time, thus generated a warning).

This should be killed once

  https://github.com/sphinx-contrib/httpdomain/pull/19

is accepted and released.

the env var setup and django.setup() needs to be done only once.

so we have more than just the 's' index entry 's' in there.

- 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.

so that we can have some generic substitutions which can be used consistently
in the doc.

in doing so, switch to the current, non-obsolete format for specifying
intersphinx mappings

hack to set the adequate settings module based on which project doc is built),
activate extlinks extension for swh-web and add links to swh projects dedicated doc
on the index page but also a link to the swh urls index

note: this was enabled only for swh-web, but it really needs to also be in
swh-docs, because we pull there docs from individual modules

Django wants to know the path to its configuration file at import time, which is
somewhat annoying when using autodoc on a Django project such as swh.web.

Close T763.

Before this change we were using the versionless URL
Rationale: upstream has deduplicated py2/py3 inventories.