Last minute fixes + move item collection to fragments

[m-mohr]

Related Issue(s): #

Proposed Changes:

1. Add OpenAPI descriptions to please CI
2. Some other improvements for OpenAPI found in the HTML versions from Matt (e.g. summary for OAFeat's 'GET /')
3. Moved itemcollection readme and examples to the correct folder (why was it in core?)

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.
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.

[cholmes]

Moved itemcollection readme and examples to the correct folder (why was it in core?)

It felt more 'core' to me than the other fragments, as it was its own 'spec' in stac-spec. I don't feel that strongly though, so it's fine in its new spot.