Make spec "verifiable document" generic

[nissimsan]

Continuing from #8, this group should focus just on business data modeling, use cases, and policy - not attempt to define technical specifications. I suggest we don't list concrete formats on https://uncefact.github.io/spec-untp/docs/specification/VerifiableCredentials, but rather describe the general 3-party model, and let that settle in the technical communities where it belongs.

[onthebreeze]

very happy to leave technical stuff to technical communities. But we also need to define something concrete and implementable. What about making explicit references to existing technical community work like the DHS SVIP interop profile - and all we do in the UN spec is say that technical implementations of DPP, conformity credentials, etc etc SHOULD (or MUST?) conform to the reference technical interop profile/version?.

[nissimsan]

Closed with
#60