add extensions section to ocgapi-features, cleanup other extensions s…

[philvarner]

Related Issue(s): #180

Proposed Changes:

1. Add Extensions section to STAC API - Features definition. I'm not sure this was linked to from anywhere prior.

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.

[cholmes]

This looks good. Note that we also probably at some point should define our OGC API - Features extensions for sort and fields. Since I'd guess many STAC Servers actually use those. At some point in the future we'll switch over to what OGC is doing, but it'd be good to officially bind our 'fragments' to the features endpoints, as that was the whole idea of defining them as 'fragments', we just never got to actually making the real ogcapi-features extensions versions of them.