Change root endpoint (/) the be a Catalog, rather than a Collection or a Catalog

[philvarner]

Right now, the text spec just says that it's JSON, but the OpenAPI definition says it can be either a Catalog or a Collection. I think it would be cleaner if this was only a Catalog. Allowing it to be either makes it a bit more complex to interact with, and I feel that generally an API will be built over a Catalog rather than a single Collection.

[cholmes]

+1 - makes sense to me. Would be weird if it's just a collection, and if an implementor wants to do just have one collection they can just put a catalog that links to one collection.

[matthewhanson]

Agree as well, Catalog requirement makes sense.

[philvarner]

I'm going to add this to the description of Catalog in stac-spec and put a PR in to update this repo.