New React Native Landing Page

Summary:
The motivation is that we haven't changed the copy on the initial React Native landing page since launching, and we have a much clearer view of the React Native value prop now.

Themes:
1. React Native is like React but for mobile apps
2. A React Native app is a "real native app"
3. Development is fast
4. You can drop down to normal native development if you need
Closes #8291

Differential Revision: D3466855

Pulled By: JoelMarcey

fbshipit-source-id: d1a5035640bcd795704d5f830b79e7c3d2e3ab02

website/src/react-native/css/react-native.css

@@ -377,7 +377,7 @@ h1:hover .hash-link, h2:hover .hash-link, h3:hover .hash-link, h4:hover .hash-li
 }
 
 .hero {
-  background: #05A5D1;
+  background: #2D2D2D;
   padding: 50px 0;
   color: #FDF3E7;
   font-weight: 300;