docs: describe environment override feature (#21879)

docs/1.getting-started/3.configuration.md

@@ -29,6 +29,27 @@ Every configuration option is described in the [Configuration Reference](/docs/a
 You don't have to use TypeScript to build an application with Nuxt. However, it is strongly recommended to use the `.ts` extension for the `nuxt.config` file. This way you can benefit from hints in your IDE to avoid typos and mistakes while editing your configuration.
 ::
 
+### Environment overrides
+
+You can configure fully typed, per-environment overrides in your nuxt.config
+
+```ts [nuxt.config.ts]
+export default defineNuxtConfig({
+  $production: {
+    routeRules: {
+      '/**': { isr: true }
+    }
+  },
+  $development: {
+    //
+  }
+})
+```
+
+::alert{type=info}
+If you're authoring layers, you can also use the `$meta` key to provide metadata that you or the consumers of your layer might use.
+::
+
 ### Environment Variables and Private Tokens
 
 The `runtimeConfig` API exposes values like environment variables to the rest of your application. By default, these keys are only available server-side. The keys within `runtimeConfig.public` are also available client-side.