[JSON-LD context] Permanent IRI for the context

[RubenVerborgh]

The current context has https://openknowledgebe.github.io/open-education/api/context.json as IRI. We probably want something more stable such as w3id, preferably without the .json suffix as to not give the impression that negotiation is excluded.

[jhoobergs]

I have already done w3id, but I was currently waiting for them to mergen my new PR so I gave you the raw url