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>