[IMP] doc: adapt guidelines for 12.0

Odoo evolves and its coding guidelines too.
This commit :
 - put some news guidelines, case to take
 into account when making new dev
 - update some guidelines that are difficult
 to applied in the real life
 - remove deprecated stuff (RML report, ...)
 - remove references to old
 api mainly, since all new dev must be done in
 new api

Task #1817988