add link relation table to each conformance class

[philvarner]

Related Issue(s): #142

Proposed Changes:

1. Add table with Link Relations to each conformance class to clarify which ones should be implemented.

PR Checklist:

• This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• This PR has no breaking changes.
• This PR does not make any changes to the core spec in the stac-spec directory (these are included as a subtree and should be updated directly in radiantearth/stac-spec)
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.

[matthewhanson]

Why have the duplicated list of link relations that should be at the root catalog listed under the other endpoints?

[philvarner]

Why have the duplicated list of link relations that should be at the root catalog listed under the other endpoints?

The idea was to make them all independent and self-contained definitions. I think what i need to do is take the common ones that apply to all of them and put them somewhere else, and then reference them, then, only the ones specific to each class need to go into the api definition.