Revise documentation of cog_default in collection summaries

[anayeaye]

What

STAC collections now support multi asset items and no longer enforce cog_default assets be included in each item. Cog_default summaries have been removed from the automated dataset summaries as well (while still valid when present, summaries of cog_default statistics are no longer guaranteed in all collections). Review documentation to clarify.

Note

Render metadata are now included in most of the collections in veda-data and these visualization hints may be useful when updating docs.