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": "33013ed3-dcfb-4edd-8c42-0fdd69f169ec",
    "type": "retrospective_process_group_steps",
    "attributes": {
      "retrospective_process_group_id": "25c04399-2d02-4f21-9400-442557adff7e",
      "retrospective_step_id": "f340fd1d-1f0a-4b9f-bdf4-8d0faaff7f0a",
      "position": 1,
      "updated_at": "2025-12-02T16:17:00.508-08:00",
      "created_at": "2025-12-02T16:17:00.508-08:00"
    }
  }
}