docs: document fallback prop for <NuxtLayout> (#26091)

docs/3.api/1.components/3.nuxt-layout.md

@@ -55,6 +55,10 @@ Please note the layout name is normalized to kebab-case, so if your layout file
 Read more about dynamic layouts.
 ::
 
+- `fallback`: If an invalid layout is passed to the `name` prop, no layout will be rendered. Specify a `fallback` layout to be rendered in this scenario. It **must** match the name of the corresponding layout file in the [`layouts/`](/docs/guide/directory-structure/layouts) directory.
+  - **type**: `string`
+  - **default**: `null`
+
 ## Additional Props
 
 `NuxtLayout` also accepts any additional props that you may need to pass to the layout. These custom props are then made accessible as attributes.