Add more info on sizes / formats for thumbnail, overview and visual

[cholmes]

I thought we agreed on having a chapter that defines recommended sizes and formats for thumbnail, overview and visual? maybe don't say a lot in the table and add a link there to this chapter while extending this chapter with recommendations about thumbnails, overviews etc. in general?

_Originally posted by @m-mohr in https://github.com/radiantearth/stac-spec/pull/989#discussion_r581923245_

[cholmes]

I forgot our recommendations - did a bit on thumbnail, but didn't get a chance to do more. Matt, if you can come up with the recommendations I'm happy to make the PR, or you just can.

[m-mohr]

Trying to remember... was it:

• thumbnail: <600x600, png/jpeg/webp, rgb
• overview: full res, png/jpeg/webp, rgb
• visual: full res, any file format (cog preferred), rgb
• (data, full res, any file format, all bands/bands per file)

@matthewhanson ?

[emmanuelmathot]

@m-mohr , I do not see the difference between overview and visual in your list except the format. IMO overview is a high definition browse image of the dataset, something that can be easily displayed on a map without tiling (e.g. overlay, kml) for which a size limit may be recommended. visual is the full res and thus often requires to have tiled images (recommended cog).

[m-mohr]

Yes, indeed, there's not much difference. It's just what I recall from a discussion, not necessarily a proposal from my side. We can figure out the file types from the type as well, so if we can't find a good enough difference, we should drop one of the roles again.

[cholmes]

I think there's decent value in 'overview', and I like the use case you're suggesting @emmanuelmathot

The common place I see this is with commercial satellite data - a 'try before you buy'. You can get a thumbnail and the overview to see what the data is about, but you have to purchase to get the full resolution.