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.
Related to #4496 (closed)