DPP example update
Created by: nissimsan
This PR updates the DPP example substantially:
- adds example values instead of types (like "string")
 - aligns the credential to the VC data model
 - adds a bunch of LD (assumes "id": "@id" and "type": "@type")
 - exemplifies a referenced context
 - links to conformity credentials instead of having them embedded
 
Pls consider this incremental improvement, I'm not suggesting this to be a final model at all; we have much more to discuss and improve on this still.