spec: Update DPP data model based on working changes from 564

The DPP documentation is updated (well, partly reverted) to use the Product type for the credentialSubject. I generated this using a combination of the diff from !564 (merged) and part of the diff from @ashley.harwood 's 0.6.0 doc change.

It also updates the data model svg and a few other small improvements (separating the core product properties a little from the DPP ones).

I assume the link to the actual example on showthething.com can only be updated when we've created one (which is probably only after releasing? Not sure).