Test that collections adhere to the specifiction
Implement collections specification
Remove obsolete available statuses endpoint

With the availibility of those statuses in the WP form this became obsolete.
Merge pull request #2231 from opf/fix/17403_implement_collection_specification

17403 implement collection specification
fix examples

- status name was removed long ago
- Project endpoint is neither implemented nor specified like the example

[ci skip]
specify category properties

[ci skip]
specify self-endpoint for category
link category of WP

[ci skip]