docs: add missing step migrating to pinia (#20778)

docs/7.migration/2.configuration.md

@@ -142,6 +142,16 @@ Install the [@pinia/nuxt](https://nuxt.com/modules/pinia) module:
 yarn add pinia @pinia/nuxt
 ```
 
+Enable the module in your nuxt configuration:
+
+```ts [nuxt.config.ts]
+import { defineNuxtConfig } from 'nuxt/config';
+
+export default defineNuxtConfig({
+  modules: ['@pinia/nuxt']
+})
+```
+
 Create a `store` folder at the root of your application:
 
 ```ts [store/index.ts]