Add archived case study example

Note this includes commits from #3 so it will only have one commit once #3 has been merged
Apologies, the proposal was out of date. The publishing [actual code](https://github.com/alphagov/whitehall/blob/77b325479343a3d09536af843e7eb5b1903afb05/app/presenters/publishing_api_presenters/case_study.rb#L25) and the [schema](https://github.com/alphagov/govuk-content-schemas/blob/master/formats/case_study/publisher/schema.json#L144) use `"archive_notice"` here.