docs: update hero image on index page

- Remove logo image (flutter-news-app-full-source-code-logo.png)
- Add screenshot image (flutter-news-app-full-source-code-screenshot.png)
- Update hero image reference in index.mdx

Author: fulleni

src/assets/flutter-news-app-full-source-code-logo.png

@@ -5,7 +5,7 @@ template: splash
 hero:
   tagline: Includes a Flutter mobile app, a web-based dashboard, and a powerful Dart Frog backend.
   image:
-    file: ../../assets/flutter-news-app-full-source-code-logo.png
+    file: ../../assets/flutter-news-app-full-source-code-screenshot.png
   actions:
     - text: 'Local Setup'
       link: /docs/getting-started/local-setup/