Add collection thumbnails to all collections

All STAC collections should have thumbnails, so they render more nicely e.g. STAC Browser. A role model here is Planetary Computer, where catalog thumbnails are loaded from STAC.

How to select thumbnails, in order of priority:

If the dataset is configured in the VEDA Dashboard:

1. Use a representative rendered layer screenshot (use the zoom level that shows the main features of the dataset, e.g. a continent if that is the scale the spatial variation occurs at)
2. The configured thumbnail from the web catalog

If the dataset is NOT configured in the VEDA Dashboard:

1. Ask the data provider to create a thumbnail
2. If you have enough expertise to inform what a good visualization could look like, open an asset in QGIS or some other client, pick a good color palette and stretch range, create thumbnail, submit in a separate PR for team review.

Acceptance criteria

• All collections have thumbnails
• Thumbnails are somehow stored and published from this repo

No files or tests are named. Start by inventorying the repository's STAC collections and checking how collection metadata is stored and published, then review the VEDA Dashboard configuration for available thumbnails. Done means every collection has a suitable thumbnail that is stored in and published from this repository.