Example .stencil file that maps a custom category template called react-demo.html to the page url /shop-all/

.stencil

{
    "normalStoreUrl": "https://<store-domain>",
    "accessToken": "<api-token>",
    "port": 3000,
    "customLayouts": {
      "brand": {},
      "category": {
        "react-demo.html": "/shop-all/"
      },
      "page": {},
      "product": {}
    }
  }