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": "89c126b7-b55a-4830-8bf5-d62a0fb861a3",
    "type": "retrospective_process_groups",
    "attributes": {
      "retrospective_process_id": "5778791e-be8c-4f16-8324-c362b03ce81a",
      "sub_status_id": "8510f797-b183-46aa-ae63-d53f3d90bc01",
      "position": 3,
      "updated_at": "2025-06-19T02:43:13.330-07:00",
      "created_at": "2025-06-19T02:43:13.330-07:00"
    },
    "relationships": {
      "retrospective_process_group_steps": {
        "data": []
      }
    }
  }
}