Context:
The API implemented with OPT-6531 needs to be added to the backup docs section (https://docs.camunda.io/docs/self-managed/backup-restore/optimize-backup/)

AT:

• get backup state API docs are updated
  • includes path
  • explains bakcupID is optional
  • includes response codes
  • includes example request
  • includes example response body
  • heading is updated since API no longer just returns state