docs: add router.extendRoutes example to migration docs (#4023)

* docs: add `router.extendRoutes` example to migration docs

* style: lint

docs/content/3.docs/3.migration/2.configuration.md

@@ -13,23 +13,53 @@ Nuxt configuration will be loaded using [`unjs/jiti`](https://github.com/unjs/ji
 
 ### Migration
 
-You should migrate to the new `defineNuxtConfig` function that provides a typed configuration schema.
+1. You should migrate to the new `defineNuxtConfig` function that provides a typed configuration schema.
 
-::code-group
+   ::code-group
 
-```ts [Nuxt 2]
-export default {
+   ```ts [Nuxt 2]
+   export default {
      // ...
-}
-```
+   }
+   ```
 
-```ts [Nuxt 3]
-import { defineNuxtConfig } from 'nuxt3'
+   ```ts [Nuxt 3]
+   import { defineNuxtConfig } from 'nuxt3'
    
-export default defineNuxtConfig({
+   export default defineNuxtConfig({
      // ...
-})
-```
+   })
+   ```
+
+   ::
+
+1. If you were using `router.extendRoutes` you can migrate to the new `pages:extend` hook:
+
+   ::code-group
+
+   ```ts [Nuxt 2]
+   export default {
+     router: {
+       extendRoutes (routes) {
+         //
+       }
+     }
+   }
+   ```
+
+   ```ts [Nuxt 3]
+   import { defineNuxtConfig } from 'nuxt3'
+   
+   export default defineNuxtConfig({
+     hooks: {
+       'pages:extend' (routes) {
+         //
+       }
+     }
+   })
+   ```
+
+   ::
 
 #### ESM Syntax