feat(server): support comment notification type (#12924)

#### PR Dependency Tree


* **PR #12924** 👈
  * **PR #12925**

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
* Introduced comment and comment mention notifications, including email
notifications when users are mentioned or receive comments on documents.
* Added new email templates for comment and comment mention
notifications.
* Users can now control whether they receive comment-related emails via
a new user setting.

* **Bug Fixes**
  * None.

* **Documentation**
* Updated GraphQL schema documentation to reflect new notification types
and user settings.

* **Refactor**
* Streamlined and enhanced test coverage for notification and user
settings, including comment notifications.

* **Chores**
* Improved test setup and snapshot coverage for user settings and
notifications.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/mails/docs/index.ts

@@ -1 +1,3 @@
+export * from './comment';
+export * from './comment-mention';
 export * from './mention';