[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.

Empieza por trazar cómo se gestionan las modificaciones de documentos, los permisos de usuario y las preferencias; en el issue no se identifican archivos ni puntos de entrada. Aclara el canal de notificación inicial, el comportamiento de las suscripciones, el alcance de los activadores y los criterios de aceptación antes de la implementación. Done debe incluir un alcance acordado y comprobable para las notificaciones y su configuración.

Escrito por el modelo de indexación a partir del texto del issue.