Make clear that the “API reference” page is an index of our modules.
Add a pointer to the “interface” part of the landing page for people
wanting to learn how to interact with Software Heritage.

Closes #4740

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

Closes T3737