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": "4ebf4e6e-a577-4dea-8eee-3bccc0526d93",
    "type": "retrospective_process_groups",
    "attributes": {
      "retrospective_process_id": "ca13907a-d9ff-4975-bca3-3bbd4b5e6856",
      "sub_status_id": "a09e9ab8-39a1-41cf-a13e-f497e9a8fb62",
      "position": 3,
      "updated_at": "2025-08-21T23:47:00.143-07:00",
      "created_at": "2025-08-21T23:47:00.143-07:00"
    },
    "relationships": {
      "retrospective_process_group_steps": {
        "data": []
      }
    }
  }
}