Don't repeat behaviors/endpoints defined in Core in each of the other conformance classes

[philvarner]

e.g., every CC has the service-desc and service-doc description, even though it's defined in core. Just reference it instead of duplicating the definition each time.

[philvarner]

@cholmes I remember you wanted each conformance class to be "complete", in that an implementer who wanted to implement it could look at the complete set of endpoints and link rels they would need in the spec. I think there are enough classes now that it's not unreasonable to remove the duplication and just reference Core for some things. Also, possibly remove the duplicate Collections definition in STAC API - Features and just reference STAC API - Collections?