Update or rework ddd.md

The diagrams in ddd.md have issues with null-values in the diagrams. Also they don't seem to be up to date.

A short list of findings:

• null values in diagrams
• pending_requestor_state in state-diagram missing
• null type in IdentityAggragate
• null value in IdentificationProcess Aggregate
• missing/old resource definitions
  • cancel/finish task
  • uploadbylink
  • triggerIssuance
  • getSSIInviteUrl
  • updateContact
  • createSession misses the parameter for the profile
  • some names differ

On the other hand i don't know if we need those diagrams in the final docs, since they we're mostly for development. I startet a small branch for this !180 (merged).