docs(pages): add information for typing page metadata (#5270)

Co-authored-by: Daniel Roe <daniel@roe.dev>

docs/content/2.guide/3.directory-structure/10.pages.md

@@ -287,6 +287,21 @@ You can define transition properties for the `<transition>` component that wraps
 
 You can define page aliases. They allow you to access the same page from different paths. It can be either a string or an array of strings as defined [here](https://router.vuejs.org/guide/essentials/redirect-and-alias.html#alias) on vue-router documentation.
 
+### Typing Custom Metadata
+
+If you add custom metadata for your pages, you may wish to do so in a type-safe way. It is possible to augment the type of the object accepted by `definePageMeta`:
+
+```ts [index.d.ts]
+declare module '#app' {
+  interface PageMeta {
+    pageType?: string
+  }
+}
+
+// It is always important to ensure you import/export something when augmenting a type
+export {}
+```
+
 ## Navigation
 
 To navigate between pages of your app, you should use the [`<NuxtLink>`](/api/components/nuxt-link) component.