This change reorganizes internal files so the Discuss-related code can be separated from Mail in a future update. For business users, there should be no immediate change in behavior, but it supports cleaner product structure and easier maintenance going forward.

*: documents

In preparation of splitting 'discuss' and 'mail' modules.

This allow using new folder discuss/ that contains all static files of new module 'discuss'.

Part of task-3265211

https://github.com/odoo/odoo/pull/119860