Update architecture diagram to show notifications system

[matt-lebl]

Update the high-level architecture (Mental Model) for the application you made as a development team. This architecture must show the new changes asked by the Clients.

• [x] Update diagram
• [x] Write diagram description (Moved to #37)

[matt-lebl]

The client team sent us an RFC, and we can update the architecture diagram to include the notification system they asked for:

Notifications: We want to implement a notification system that will notify users when they receive new messages, notifications about marks, comments on assignments. This will help to keep users engaged and informed about activity on the platform.

This can be its own standalone system in the backend bubble, and it needs to communicate with some external messaging systems like email, SMS, etc.