Clarify what should go in 'child' links and if they're required, etc.

[cholmes]

Most implementors get pretty confused as to what's supposed to go in 'child' links, and how that relates to the data resource to collections/.

The child links are quite useful, as they'll be crawled and parsed by any STAC tool, but we should make a recommendation of what should go there. And decide if we want to require them, or have it be ok if they aren't used. A common approach seems to be to just have them be a reflection of the individual resources under 'collection'. Which does seem to have some value to be able to have it 'work' with any STAC tool, but we should make it clear why we duplicate things.

We also should explain how it can be used to provide different 'views' of the data, and can be used to browse all the way down to items.