updates to make clear root can be catalog or collection

[cholmes]

Related Issue(s): #1082

Proposed Changes:

1. Various little tweaks to make it clear that the 'root' can be a Catalog or Collection
2. Or small language tightening that came up as reading.

PR Checklist:

• [x] This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• [x] This PR has no breaking changes.
• [x] a CHANGELOG entry is not required.

[m-mohr]

Some more parts, which seem to apply to both Catalog and Collection:

best-practices (I haven't read it completely):

Sub-Catalogs should be stored in subdirectories of their parent

Items should be stored in subdirectories of their parent Catalog.

Limit the number of Items in a Catalog or sub-Catalog, grouping / partitioning as relevant to the dataset.

providing multiple 'sub-catalogs' from the root Catalog that enable different paths to browse

This is a self-contained catalog as described above, except it includes an absolute self link at the root catalog

catalog-spec:

Every STAC Collection is also a valid STAC Catalog.

Not true in validation due to the type field.

Note: A link to at least one item or child Catalog is REQUIRED.

One child link is required, can be Catalog or Collection.

collection-spec:

Summaries should summarize all values in every Item underneath the collection, including in any nested sub-Catalogs.

| parent | URL to the parent STAC entity (Catalog or Collection). Non-root Collections should include a link to their parent. | | child | URL to a child STAC Catalog or Collection. |

Why is it sometimes "STAC entity (Catalog or Collection)" and sometimes "STAC Catalog or Collection"?

[cholmes]

Weird, I had a couple of those on my local copy but didn't get them committed. Will do the rest and commit.

[cholmes]

I think I got them all.