Define where to put catalog level metadata in a STAC API

[cholmes]

For static catalogs we have fields like 'tags' and 'contact', as well as proposals like 'homepage' in #137. It is less clear where to put this type of metadata in a stac api.

Static catalogs tend to have a 'catalog' field, that refers to its root catalog that has this field. But that root catalog is also the 'starting point' to point someone at to learn about the dataset and dive in. There should be a well known place in the STAC API that has that info. It's likely either a set of json fields in /collections/{collectionId} or a little document that hangs off it, like /collections/{collectionId}/catalog.json

The latter likely makes more sense, especially if there's a nice way to have it work the same for static and dynamic catalogs.

Related to #81 as well as concepts around collection level metadata.

[cholmes]

This will be done with #172 - the conclusion was that STAC API's should have a /stac endpoint that has a catalog structure that follows the static spec.

[cholmes]

Not sure why I closed this - should be closed when #172 is fully merged and the concept is fully explained.

[cholmes]

closed with #172 and #258