Retrieves an incident retrospective

curl --request GET \
  --url https://api.rootly.com/v1/post_mortems/{id} \
  --header 'Authorization: Bearer <token>'

{
  "data": {
    "id": "55870028-0663-403f-948d-7d60deba971f",
    "type": "incident_post_mortems",
    "attributes": {
      "incident_id": "44900caa-c5c1-4941-ab4a-cfe54138628f",
      "title": "Rerum quis deserunt non.",
      "status": "draft",
      "url": "https://test.rootly.com/account/post_mortems/rerum-quis-deserunt-non",
      "short_url": null,
      "content": null,
      "published_at": null,
      "started_at": "2025-08-21T23:44:54.975-07:00",
      "mitigated_at": null,
      "resolved_at": null,
      "cancelled_at": null,
      "show_timeline": true,
      "show_timeline_starred_only": false,
      "show_timeline_genius": true,
      "show_timeline_trail": true,
      "show_timeline_tasks": true,
      "show_timeline_action_items": true,
      "show_timeline_order": "desc",
      "show_functionalities_impacted": true,
      "show_services_impacted": true,
      "show_groups_impacted": true,
      "show_action_items": true,
      "created_at": "2025-08-21T23:44:55.596-07:00",
      "updated_at": "2025-08-21T23:44:55.604-07:00"
    },
    "relationships": {
      "incident_retrospective_steps": {
        "data": [
          {
            "id": "a4ae22b2-26c6-4413-937e-6fab0a77fd21",
            "type": "incident_retrospective_steps"
          }
        ]
      }
    }
  }
}