Clarify how rel=items is implemented in the STAC API spec

[lossyrob]

Collections that are returned from an API do not have any rel=item links, but have rel=items. The meaning of this reltype is inherited by the OGC API: Features spec. However a STAC user trying to understand the meaning of rel=items vs rel=item has to dig in a few different places to piece together how those two link types (don't) relate, and that rel=items isn't part of core STAC but instead the STAC API by way of oafeat.

This topic could use some documentation in the STAC API spec, and potentially in the Collection spec, to make clear where rel=items comes from.

See this Gitter conversation for a discussion about this topic.

[cholmes]

My comments on #159 are also related to this - I think we also need to get more clear on how things should be done in API specs.

[matthewhanson]

Does there need to be an additional conformance class for this? Or perhaps that's already handled by the OAF conformance. It just doesn't define a rel=items link type.
There's a conformance class for item-search, but that only applies to the search.

[philvarner]

I added some clarifying language here: #172

A better place for this will be when we add the Collections conformance class #134