When there is no search query, show featured collections

Featured collections at the beginning of the doc list when there is no search query could be a simple way to get people to interact with the documents.

These "collections" could point to:
1. A project filter search that gets loaded as the doc list below,
2. The collection's own briefcase instance,
3. An existing website/webpage/blog link.
4. A post/page within briefcase that gives detail to the collection with a custom embed of the doc set.

The collection feature should:
1. Include a thumbnail image + title + short description,
2. Be a sliding carousel using [Slick](https://kenwheeler.github.io/slick/)