Update UML diagrams for r1.2

[stroncoso-quobis]

Problem description
UML diagrams were done for tag v1.1.0.
https://github.com/camaraproject/WebRTC/tree/main/documentation/API_documentation/uml-diagrams

Expected action
Update to fit with r1.2:

• Remove notification channel UML
• Update registration and call-handling
• Include diagram for events API

Additional context
UML diagrams are a valuable asset for vendor manufaturers, as it states and clarify a possible interaction with the network functions. Also, it helps to clarify how the API works and could be linked from inline YAML to improve the APIs documentation itself.