docs: fix Metro bundler startup instructions (#2795)

## Summary

This PR fixes the misleading documentation in `getting-started.md` that
incorrectly stated Metro bundler would open automatically when running
`npx react-native run-macos`.

As confirmed in issue #2208, Metro bundler must be started separately
before running the app and this is expected behavior by design. Users
following the current documentation were encountering the error "**_No
script URL provided. Make sure the packager is running or you have
embedded a JS bundle in your application bundle_"** because they
expected Metro to start automatically.

## Changes

- Updated the Quick Start section to include Metro bundler startup as
the first step
- Removed the incorrect statement: "A Metro bundler window will also
open to serve your JavaScript code"
- Clarified that Metro must be running in a separate terminal before the
app can connect to it

Fixes #2208

Author: alejorrojas

docsite/docs/getting-started.md

@@ -39,13 +39,21 @@ npx react-native-macos-init
 
 ### Quick Start
 
-In your React Native macOS project directory, run:
+1. **Start the Metro bundler** in your React Native macOS project directory:
+
+```bash
+npm run start
+```
+
+Keep this terminal window open. The Metro bundler must be running for your app to load JavaScript code.
+
+2. **Build and launch your app** in a separate terminal window:
 
 ```bash
 npx react-native run-macos
 ```
 
-This will build your app and launch it automatically. A Metro bundler window will also open to serve your JavaScript code.
+This will build your app and launch it automatically.
 
 ### Alternative Methods