feat: update project logo and assets

- Replace hero image from houston.webp to flutter-news-app-full-source-code-logo.png
- Update is reflected in the src/content/docs/index.mdx file

Author: fulleni

src/content/docs/index.mdx

@@ -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/houston.webp
+    file: ../../assets/flutter-news-app-full-source-code-logo.png
   actions:
     - text: 'Get Started: Local Setup'
       link: /getting-started/