Creates a retrospective template

curl --request POST \
  --url https://api.rootly.com/v1/post_mortem_templates \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/vnd.api+json' \
  --data '
{
  "data": {
    "type": "post_mortem_templates",
    "attributes": {
      "name": "<string>",
      "content": "<string>",
      "default": true,
      "format": "html"
    }
  }
}
'

{
  "data": {
    "id": "02b3ed35-a4f8-431c-bc38-6babf45ec4bf",
    "type": "post_mortem_templates",
    "attributes": {
      "name": "My Retrospective Template",
      "slug": "my-retrospective-template",
      "format": "markdown",
      "default": false,
      "content": "## Test template\n\n### Test new line",
      "created_at": "2025-12-18T18:59:51.110-08:00",
      "updated_at": "2025-12-18T18:59:51.112-08:00"
    }
  }
}