docs: add mention of addServerTemplate to modules guide (#31369)

docs/2.guide/3.going-further/3.modules.md

@@ -579,6 +579,22 @@ export default defineNuxtModule({
 })
 ```
 
+For the server, you should use the `addServerTemplate` utility instead.
+
+```ts
+import { defineNuxtModule, addServerTemplate } from '@nuxt/kit'
+
+export default defineNuxtModule({
+  setup (options, nuxt) {
+    // The file is added to Nitro's virtual file system and can be imported in the server code from 'my-server-module.mjs'
+    addServerTemplate({
+      filename: 'my-server-module.mjs',
+      getContents: () => 'export const myServerModule = () => "hello world !"'
+    })
+  }
+})
+```
+
 #### Adding Type Declarations
 
 You might also want to add a type declaration to the user's project (for example, to augment a Nuxt interface