add recommendations about STAC Items/Collections extensions

[vincentsarago]

in https://github.com/vincentsarago/MAXAR_opendata_to_pgstac/discussions/2 I've added the item-assets extension to the collections and then I'm using it (if available) in the mosaic-builder https://github.com/developmentseed/eoAPI/blob/3dd87b806190c420323749b76b2cdaf9957fecec/runtime/eoapi/raster/eoapi/raster/templates/mosaic-builder.html#L451-L469

Collections

• item-assets ( https://github.com/stac-extensions/item-assets)

Items

• eo
• projection
• alternate-assets
• ?