also, use anchors to link to modules rather than relying on generated HTML file
names

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

Build prior to swh.web module.  It's a workaround the time to find a
more sustainable solution.

Related T763

This is needed because the multiplexer makefile that build apidoc in all
modules explicitly call "make apidoc". Also, it is reasonable to do nothing
anyhow in that case if SKIP_APIDOC=yes is explicitly set.

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

Makefile.sphinx: also exclude root tests module for apidoc as it may have content defined in its __init__.py (like in swh-web for instance)

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.

Works around T763 until a generic solution is found

purely cosmetic change

This allows you to still have python-sphinx and python3-sphinx coinstalled and
still build with Python 3.

... previously shipped as part of swh-environment

Closes T751

document how to publish the docs in README.md

use case: swh-storage

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