In odoo/documentation-user#945, the developer documentation is merged
with the user documentation in a single repository and build config.

This commit then removes source files for the developer documentation
from the odoo/odoo repository.

DOC PR: https://github.com/odoo/documentation-user/pull/945

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

Co-authored-by: Victor Feyens <vfe@odoo.com>
Co-authored-by: Antoine Vandevenne <anv@odoo.com>

A comprehensive theme’s revamp, details:

=== New features ===
- Show titles permalink markers on hover
- “Scrollspy” effect for the sidebar

=== Fixes ===
- Solve an issue causing the sidebar to flicker on scroll
- Hide the sidebar if less then two links

=== Design ===
- Improve typography vertical rhythm and line height
- Visually increase separation between sections
- Review alerts and <dl> design
- Reduce cover’s size
- Reduce overall paddings/margins.

Closes #21304

* fix Sphinx 1.6 compatibility leading to the "tiles" on the home page
  not working anymore: 1.6 replaces BuildEnvironment.reolve_toctree by
  TocTree().resolve(), while the method still exists it's not actually
  called anymore
* rejigger some CSS as the second section went from one big tile to 3
  smaller tiles, and got laid out as a row rather than a second 2x2
  block. Recode the entire mess with flexbox, remove some stuff which
  conflicted with boostrap (this screen should probably be
  de-boostrapped and completely converted to flexbox or grid)
* add building CSS from LESS to the makefile, all Odoo devs should
  have less installed locally (for assets)

IAP allow app publishers to charge for ongoing services. In that context, Odoo
acts mostly as a payment platform between the service user (client) and the service
provider (Odoo App developer).