Clarify plugin installation in index.md

Updated comments to clarify plugin installation and features.

Signed-off-by: Vincenzo Scamporlino <vincenzos@spotify.com>

docs/frontend-system/building-apps/01-index.md

@@ -34,13 +34,13 @@ This is how to create a minimal app:
 ```tsx title="in src/index.ts"
 import ReactDOM from 'react-dom/client';
 import { createApp } from '@backstage/frontend-defaults';
-import catalogPlugin from '@backstage/plugin-catalog/alpha';
 import '@backstage/ui/css/styles.css';
 
 // Create your app instance
 const app = createApp({
-  // Features such as plugins can be installed explicitly, but we will explore other options later on
-  features: [catalogPlugin],
+  // Custom features such as plugins can be installed explicitly, but they are usually
+  // auto-discovered, unless `app.packages` app-config is customized.
+  features: [],
 });
 
 // This creates a React element that renders the entire app