Display example evalscript for each collection in YML file

[tristaneo]

No description provided.

[dthiex]

@tristaneo We already list evalscripts for a lot of different collections (e.g. here for Landsat7 true color). Is there something specific you were missing?

If the evalscript for the collection exists in our public custom script repository it can be easily added to the collection (which is on SH).