Docs: `color` option for preview icon in sections

[bnomei]

https://getkirby.com/docs/reference/panel/sections/pages#preview-images

image:
  query: page.image.findBy("name", "cover")
  cover: true
  ratio: 1/1
  back: black
  color: red # <-- undocumented

possible values... color: black color: '#BADA55' color: '{{ page.panelIconColor }}' query string with custom page method works as well