Refining JSON-LD structure

A few improvements to the `@context` such as adding `@language` definition for `label` and prefix for dbpedia.

In the main body
- I made all values that should be a reference into an object with `@id` reference plus a label (in Dutch).
- Added `@id` for the API response and the vehicle itself which will be resolved relative to wherever the JSON is served from.
- Changed all "N.v.t" and "Niet geregistreerd" values to `null`