docs: add documentation for using layers with private repos

docs/1.getting-started/9.layers.md

@@ -29,6 +29,19 @@ export default defineNuxtConfig({
 })
 ```
 
+You can also pass an authentication token if you are extending from a private GitHub repository:
+
+```ts twoslash [nuxt.config.ts]
+export default defineNuxtConfig({
+ extends: [
+ // per layer configuration
+ ['github:my-themes/private-awesome', { giget: { auth: process.env.GH_TOKEN } }]
+ ]
+})
+```
+
+This uses [giget](https://unjs.io/packages/giget) under the hood for authenticating access to private repositories. For more information about this, visit [the documentation](https://unjs.io/packages/giget#providing-token-for-private-repositories) on "Providing token for private repositories"
+
 ::read-more{to="/docs/guide/going-further/layers"}
 Read more about layers in the **Layer Author Guide**.
 ::