Creates a retrospective process group step

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

{
  "data": {
    "id": "1fb2bbd5-22c1-4cb3-833e-06317e8bc0d6",
    "type": "retrospective_process_group_steps",
    "attributes": {
      "retrospective_process_group_id": "c7331e3c-a3d5-4a25-8705-bdd05f3ea8cd",
      "retrospective_step_id": "b6f51feb-f2f2-4991-accd-770b20316ea1",
      "position": 1,
      "updated_at": "2025-12-24T09:27:46.810-08:00",
      "created_at": "2025-12-24T09:27:46.810-08:00"
    }
  }
}