Creates a retrospective process group

curl --request POST \
  --url https://api.rootly.com/v1/retrospective_processes/{retrospective_process_id}/groups \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/vnd.api+json' \
  --data '{
  "data": {
    "type": "retrospective_process_groups",
    "attributes": {
      "sub_status_id": "<string>",
      "position": 123
    }
  }
}'

{
  "data": {
    "id": "30d2afee-4e44-4a40-82e9-c2027172def4",
    "type": "retrospective_process_groups",
    "attributes": {
      "retrospective_process_id": "6f2cf8db-d606-4985-bf9a-100c2a357bd3",
      "sub_status_id": "95ac0ce2-6d4e-43fb-8828-b13f0eca75ca",
      "position": 3,
      "updated_at": "2025-10-01T06:20:15.604-07:00",
      "created_at": "2025-10-01T06:20:15.604-07:00"
    },
    "relationships": {
      "retrospective_process_group_steps": {
        "data": []
      }
    }
  }
}