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": "7fbd7a2c-3c1a-458c-91e9-fd56a0cc9013",
    "type": "retrospective_process_groups",
    "attributes": {
      "retrospective_process_id": "8d32475d-a5b3-46e2-986d-4bc6c548073d",
      "sub_status_id": "34af27b0-028c-4f68-a9f2-43b889fb790d",
      "position": 3,
      "updated_at": "2025-09-10T23:03:57.173-07:00",
      "created_at": "2025-09-10T23:03:57.173-07:00"
    },
    "relationships": {
      "retrospective_process_group_steps": {
        "data": []
      }
    }
  }
}