Represent content that's no longer available

**needs to be updated with the correct version of `gds_api_adapters` once https://github.com/alphagov/gds-api-adapters/pull/243 is merged**

www.agileplannerapp.com/boards/173808/cards/8116

Implementing [Gone items](https://github.com/alphagov/content-store/blob/master/doc/gone_item.md) in content-store to allow representing items that are no longer available. Also updates related documentation.
I don't think we should constrain the update_type.  I think in the docs it's just an example, and not a requirement.  I can see use-cases where we would want other types (eg republish in the case of a publishing tool that has a task to repush everything to the content-store)