Refs. swh/infra/sysadm-environment#4813

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

Related to T2400

Related to T3717

Related to T3658

underscore-backtick was used instead of backtick-underscore to terminate links,
which is invalid rst syntax

It's mostly finished, remains 7 concurrent ingestion which have been running for weeks
now.

Related to T3338

Related to T3651

Related to T3568

Related to T3597

Related to T3600

Related to T3581

Related to T3374

While T735 is the upper level task, that might make curious people follow through too
much indirection. So simplifying to target directly the interesting and up-to-date task.

Related to T3374

Related to T3374

this is as reasonably done as it could be at the moment

Closes T2793

n/t to @ardumont for the help in digging the historical record

Related T2793

Related T2793

Related T2793

include a first entry about ongoing crawl pause

Closes T2460