[documentation] Create technical / workflow diagrams

[zelgerj]

Architecture as well as workflows tend to be very complex within the appserver.io environment. We need diagrams for better documentation and overview of architecture and request/response flow.

UAC:

• There MUST be workflow diagrams showing the request/response flow
• Diagrams MUST illustrate the difference in between servlet requests and legacy ones
• There MUST be architectural diagrams
• At least one diagram MUST illustrate different thread contexts
• At least one diagram MUST illustrate the architectural environment contexts