Design doc: new notification system (#10890)

* Design doc: new notification system

Initial design document to discuss the implementation of a new notification system.

* Update Sphinx titles

* Don't consider email/webhook notifications

* Clarify non-goal

* Add some queryset filters to the API

* Add `db_index`

* Add note about notification create endpoint

* Sphinx style

* Add `Message.icon_style` (solid, duotone)

* Use `news` instead of `bell`

* Remove `color` field since it's based on `type`

* Use `message_id` instead of `slug`

* Use `CANCELLED` for `state` when auto-removed

* Expand backward compatibility