Update documentation (Points 3 & 6.2)

[Drastur]

As mentioned in the revision, we must modify two sections of the arc42 documentation:

• Include inside 3.2 the external APIs and libraries used updating the diagram that is currently shown.

• Modify diagram in 6.2 to represent a pure activity diagram in which we don't mix components of our application with objects/instances created. Maybe change it for a sequence diagram. (Check the documentation of arc42)