docs(configuration): add --envName flag

Documentation accompanying the PR for Nuxt CLI: nuxt/cli#424 (currently in review)

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

@@ -42,10 +42,17 @@ export default defineNuxtConfig({
   },
   $development: {
     //
-  }
+  },
+  $myCustomName: {
+    // 
+  },
 })
 ```
 
+To select an environment when running a Nuxt CLI command, simply pass the name to the `--envName` flag, like so: `nuxi build --envName myCustomName`.
+
+To learn more about the mechanism behind these overrides, please refer to the c12 documentation on [environment-specific configuration](https://github.com/unjs/c12?tab=readme-ov-file#environment-specific-configuration).
+
 ::tip{icon="i-ph-video-duotone" to="https://www.youtube.com/watch?v=DFZI2iVCrNc" target="_blank"}
 Watch a video from Alexander Lichter about the env-aware `nuxt.config.ts`.
 ::