Add unpublishing format publishing schema/example

This adds the "unpublishing" format schema and an example. Mainly so that I can use the example when running tests whilst trying to deliver: https://trello.com/c/o1EFKTaj/60-government-frontend-handles-unpublishing-format-content-items

Also in support of https://trello.com/c/BQodP4gq/58-handle-unpublished-splash-pages-in-the-new-publishing-2-0-stack.