[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.

Commencez par recenser la manière dont sont gérées les modifications de documents, les autorisations des utilisateurs et les préférences ; aucun fichier ni point d’entrée n’est identifié dans l’issue. Clarifiez le canal de notification initial, le comportement des abonnements, la portée des déclencheurs et les critères d’acceptation avant l’implémentation. Done doit inclure une portée convenue et testable pour les notifications et leur configuration.

Rédigé par le modèle d'indexation à partir du texte de l'issue.