[REF][MOV] documentation apocalypse

[Feyensv]

Prior to this commit, the Odoo documentation was mainly split between
two repositories: odoo/odoo/doc and odoo/documentation-user. Some bits
of documentation were also hosted elsewhere (e.g., wiki, upgrade, ...).
This was causing several problems among which:

• The theme, config, Makefile, and similar technical resources had to
  be duplicated. This resulted in inconsistent layout, features, and
  build environments from one documentation to another.
• Some pages did not fit either documentation as they were relevant
  for both users and developers. Some were relevant to neither of the
  two (e.g., DB management).
• Cross-doc references had to be absolute links and they broke often.
• Merging large image files in the developer documentation would bloat
  the odoo/odoo repository. Some contributions had to be lightened to
  avoid merging too many images (e.g., Odoo development tutorials).
• Long-time contributors to the user documentation were chilly about
  going through the merging process of the developer documentation
  because of the runbot, mergebot, odoo-dev repository, etc.
• Some contributors would look for the developer documentation in the
  odoo/documentation-user repository.
• Community issues about the user documentation were submitted on the
  odoo/odoo repository and vice-versa.

Merging all documentations in one repository will allow us to have one
place, one theme, one work process, and one set of tools (build
environment, ...) for all of the Odoo docs.

As this is a good opportunity to revamp the layout of the documentation,
a brand new theme replaces the old one. It features a new way to
navigate the documentation, centered on the idea of always letting the
reader know what is the context (enclosing section, child pages, page
structure ...) of the page they are reading. The previous theme would
quickly confuse readers as they navigated the documentation and followed
cross-application links.

The chance is also taken to get rid of all the technical dangling parts,
performance issues, and left-overs. Except for some page-specific JS
scripts, the Odoo theme Sphinx extension is re-written from scratch
based on the latest Sphinx release to benefit from the improvements and
ease future contributions.

COM PR : odoo/odoo#70121

task-2351938
task-2352371
task-2205684
task-2352544

Co-authored-by: Victor Feyens vfe@odoo.com
Co-authored-by: Elisabeth Dickinson edi@odoo.com
Co-authored-by: Antoine Vandevenne anv@odoo.com

[AntoineVDV]

@seb-odoo 🔔