feat: Add introductory video to main docs page (#12240)

This PR adds a new introductory video to the main documentation landing
page (`index.md`). The video provides a hands-on demo of Flutter for new
users, helping them get started before diving into the codelab.

---------

Co-authored-by: Shams Zakhour (ignore Sfshaza) <[email protected]>

Author: khanhnwin

src/content/index.md

@@ -26,6 +26,13 @@ see [What's new][].**
 
 ## New to Flutter?
 
+Ready to build beautiful, multiplatform apps from a single codebase?
+This video walks you through the fundamentals of Flutter and shows you how to get started.
+
+{% videoWrapper 'Build and ship amazing multiplatform iOS and Android apps with one codebase' %}
+{% ytEmbed 'W4JWeQolJsU', 'Build and ship amazing multiplatform iOS and Android apps with one codebase' %}
+{% endvideoWrapper %}
+
 Once you've [Set up Flutter][],
 you should follow the 
 [Write your first Flutter app][] codelab