This also adds a redirection from the old path "users" to the new one "user".

Related to T3650

Related to T3658

Related to T3648

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 T3648

Related to T3648

Related to T3651

This also cleans up rendering and whitespace issues.

Related to T3154

Related to T3154

Related to T3648

Related to T3648

Related to T3648

Related to T3648

adding a docker section (stolen from swh-docker/README) and a template
for a step by step documentation.

This also adds the scaffolding to build svg out of an uml diagram.

And keep reference to official documentation if any.

toctree indentation needs to be consistent across indexes (3).