docs: add defineNuxtRouteMiddleware migration (#21718)

docs/6.bridge/1.overview.md

@@ -233,6 +233,28 @@ If you want to use the new Nuxt composables (such as `useNuxtApp` or `useRuntime
 Although a compatibility interface is provided via `nuxtApp.vueApp` you should avoid registering plugins, directives, mixins or components this way without adding your own logic to ensure they are not installed more than once, or this may cause a memory leak.
 ::
 
+## New Middleware Format (Optional)
+
+You can now migrate to the Nuxt 3 middleware API, which is slightly different in format from Nuxt 2.
+
+Middleware now take only two argument (`to`, `from`). You can find out more in [the docs](/docs/guide/directory-structure/middleware).
+
+```js
+export default defineNuxtRouteMiddleware((to) => {
+  if (to.path !== '/') {
+    return navigateTo('/')
+  }
+})
+```
+
+::alert{type=warning}
+Use of `defineNuxtRouteMiddleware` is not supported outside of the middleware directory.
+::
+
+::alert{type=warning}
+Nuxt Bridge does not support `definePageMeta`.
+::
+
 ## New `useHead` (Optional)
 
 Nuxt Bridge provides a new Nuxt 3 meta API that can be accessed with a new `useHead` composable.