Add collection thumbnails to all collections

[j08lue]

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