Add spotlight resource field notes

Add notes under each field of the spotlight configuration. I remember we were going to limit how many characters can be added for the description, but I don't remember how many we decided on, so I added ### as a placeholder.

Featured Label:

Name of the resource

Featured URL:

Link to resource (use GALILEO Express Link if available)

Featured Description:

Short description of the resource (max ### characters)

Featured Image:

Upload image (.jpg/.png) related to resource (image will be resized)