Revise documentation of cog_default in collection summaries

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.

Review the documentation that describes cog_default in collection summaries, then compare it with the collection metadata under veda-data/ingestion-data/collections. Update the documentation to explain that cog_default summaries are optional and no longer guaranteed, while noting that render metadata may provide useful visualization hints. Done means the documented behavior matches the current collections.