Use collection assets in new examples

[cholmes]

We moved collection assets to core with #1008 but the new examples refresh in #955 hadn't landed when that PR was made, so we just removed the old examples. But we should make sure that the new examples show at least one instance of a collection asset.

[cholmes]

Easiest way to do this is just add a thumbnail in one or more of the collections

"assets": {
    "thumbnail": {
      "href": "thumbnail.png",
      "title": "Preview",
      "type": "image/png",
      "roles": [
        "thumbnail"
      ]
    }
  },