Add syntax for foreign and primary keys

[tirrolo]

What's changed

Thrown in a bit of content for the structural annotations. Specifically, an example of syntax for PRIMARY KEYS and FOREIGN KEYS.

Change checklist

• [x] updated ontology, where necessary
• [x] updated shapes, where necessary
• [x] added or updated test cases, where necessary
• [x] any TODOs have been turned into trackable issues and referenced where necessary

Issue reference

[tirrolo]

@tirrolo thanks for the great work up until now. I suggested some (mostly editorial) changes. One point which I would like to discuss is the necessity of using rdf:List.

In general I agree with @elsdvlee that we should refrain from mentioning relational databases if not necessary to try to keep the text neutral. Maybe it makes sense to introduce one section on the parallel between structural annotations and relational databases?

Alright, I will need to find some time for the refactoring though.