Fix ontologies namespace; link spec to ontologies JSONLD

[VladimirAlexiev]

https://uncefact.github.io/spec-untp/docs/specification/DigitalProductPassport links to JSONLD context (which is broken, see #8).

It should also link to ontology https://jargon.sh/user/unece/DigitalProductPassport/v/working/artefacts/jsonld/render.jsonld.

• In contrast the the context, the ontology looks good (after fixing https://github.com/jargon-sh/issues/issues/3)
• However, the namespace DigitalProductPassport: https://jargon.sh/user/unece/DigitalProductPassport/v/working/artefacts/jsonld/render.jsonld# is defective: jargon.sh is a brand, and all these are parasitic words: v/working/artefacts/jsonld/render.jsonld.
  • Instead, use something like un-dpp: https://uncefact.org/untp/DigitalProductPassport/
  • I've used a shorter prefix, of course that's a matter of preference
• It would be great to establish content negotiation and semantic redirects, so that the JSONLD payload is returned