`/project/:project_id/doc/:doc_id/deleted` responds with:
- 404: the doc does not exist
- 200 and body `{"deleted":true}`: doc exists and is deleted
- 200 and body `{"deleted":false}`: doc exists and is not deleted
`/project/:project_id/doc/:doc_id/deleted` responds with:
- 404: the doc does not exist
- 200 and body `{"deleted":true}`: doc exists and is deleted
- 200 and body `{"deleted":false}`: doc exists and is not deleted