docs: add documentation for using layers with private repos

Update **Layers** documentation to include instructions for extending from private repositories.

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

@@ -29,6 +29,18 @@ export default defineNuxtConfig({
 })
 ```
 
+The above example illustrates extending from a public GitHub repository. To extend from a **private** GitHub repository, follow the modified approach outlined below.
+
+```ts twoslash [nuxt.config.ts]
+export default defineNuxtConfig({
+  extends: [
+    // per layer configuration
+    ['github:my-themes/private-awesome', { giget: { auth: process.env.GH_TOKEN } }]
+  ]
+})
+```
+
+
 ::read-more{to="/docs/guide/going-further/layers"}
 Read more about layers in the **Layer Author Guide**.
 ::