[Feature Request] - Add a notification system that informs users when a document they collaborate on or follow is modified.

**Problem**

CodiMD currently does not notify users when a document is updated. In collaborative workflows, this makes it easy to miss important changes, forcing users to manually check documents or rely on external communication tools.

**Proposed Solution**

Implement configurable notifications that are triggered when a document is modified. Notifications should respect user permissions and preferences.

**Possible delivery channels:**

- In-app notifications
- Email notifications
- Webhooks (for integrations such as Slack, Mattermost, etc.)

**Proposed Features**

- Subscribe/unsubscribe to notifications per document
- Default notification settings for document owners
- Configurable triggers (any edit, major changes only, metadata changes)

**Use Cases**

- Team members are notified when shared notes are updated
- Contributors are alerted when specifications change
- Reviewers know when a document has been edited and is ready for review

**Additional Context**

This feature would improve collaboration efficiency and make CodiMD more suitable for team-based documentation workflows.

まず、ドキュメントの変更、ユーザー権限、環境設定がどのように扱われているかを把握します。issue ではファイルやエントリーポイントが特定されていません。実装前に、初期通知チャネル、サブスクリプションの動作、トリガーの範囲、受け入れ基準を明確にします。Done には、通知とその設定について合意済みでテスト可能なスコープを含める必要があります。

索引モデルが issue の本文から書いたものです。