docs(catalog): update README for new frontend system as default

Move old frontend system wiring instructions to an "Old Frontend
System" section. The default installation path now uses package
discovery with no manual wiring needed.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

plugins/catalog/README.md

@@ -15,13 +15,17 @@ To check if you already have the package, look under
 `@backstage/plugin-catalog`. The instructions below walk through restoring the
 plugin, if you previously removed it.
 
-### Install the package
-
 ```bash
 # From your Backstage root directory
 yarn --cwd packages/app add @backstage/plugin-catalog
 ```
 
+Once installed, the plugin is automatically available in your app through the default package discovery. For more details and alternative installation methods, see [installing plugins](https://backstage.io/docs/frontend-system/building-apps/installing-plugins).
+
+## Old Frontend System
+
+If your Backstage app uses the old frontend system, you need to manually wire the plugin into your app.
+
 ### Add the plugin to your `packages/app`
 
 Add the two pages that the catalog plugin provides to your app. You can choose