Tidy up catalog references

[philvarner]

Related Issue(s): #111

Proposed Changes:

1. make explicit that / returns a Catalog and not a Collection (this is implied by the schema, but not explicit in the textual description)
2. add type field to Catalog openapi definition and example

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.

[m-mohr]

As you've added the type field for Catalog, I'd think it should also add the type field for Collections, right?