Creates a retrospective step

curl --request POST \
  --url https://api.rootly.com/v1/retrospective_processes/{retrospective_process_id}/retrospective_steps \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/vnd.api+json' \
  --data '
{
  "data": {
    "type": "retrospective_steps",
    "attributes": {
      "title": "<string>",
      "description": "<string>",
      "due_after_days": 123,
      "incident_role_id": "<string>",
      "position": 123,
      "skippable": true
    }
  }
}
'

{
  "data": {
    "id": "a4e3e3e7-38bd-4b90-bf05-b4704a46605c",
    "type": "retrospective_steps",
    "attributes": {
      "retrospective_process_id": "07449ae0-cea2-4bb2-8170-f83e3d330867",
      "title": "Step 1",
      "description": "Step 1 description",
      "incident_role_id": "ce561ae6-f87e-4539-8dbe-45e185ba33ae",
      "due_after_days": 5,
      "position": 1,
      "skippable": false,
      "created_at": "2025-12-13T18:51:57.157-08:00",
      "updated_at": "2025-12-13T18:51:57.157-08:00"
    }
  }
}