docs: add documentation for using layers with private repos (#26094)

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', { auth: process.env.GITHUB_TOKEN }]
+ ]
+})
+```
+
+Nuxt uses [unjs/c12](https://c12.unjs.io) and [unjs/giget](https://giget.unjs.io) for extending remote layers. Check the documentation for more information and all available options.
+
 ::read-more{to="/docs/guide/going-further/layers"}
 Read more about layers in the **Layer Author Guide**.
 ::