Fix the API documentation's context

Since external properties are just referenced, the "property" property needs to be type-coerced to @id.
Expose hydra:title and hydra:description for referenced external concepts

... instead of "overwriting" rdfs:label and rdfs:comment. Locally defined concepts are being described using RDFS.
Declare subClassOf relationships
Do not include properties that are exposed as keywords in the ApiDocumentation