Add diagrams to the architecture doc page

Today in the page that describes the Watcher architecture, we can only find a diagram showing the organic view of the system.

We should also add the following diagrams to provide more information about the dynamic view of Watcher :
- sequence diagrams showing what happens in Watcher for the 3 main scenarios (Create Audit Template, Launch Audit, Launch Action Plan)
- state machine diagrams that show how Audit and Action Plans state can change and what makes them change

And also some diagrams showing the data model of Watcher, especially the functional dependency of objects from the actors point of view (Goals, Audits, Action Plans, ...).