add github icon (#2878)

dotnet/website/template/public/main.js

@@ -0,0 +1,9 @@
+export default {
+    iconLinks: [
+      {
+        icon: 'github',
+        href: 'https://github.com/microsoft/autogen',
+        title: 'GitHub'
+      }
+    ]
+  }