Added new demo page.

This is a complete replacement for the old demo page, made to be more
modern-looking and easier to maintain. It contains new features such as
remembering the URIs you provide for custom assets, and searching through
the default assets by feature.
This demo page is not quite ready for release yet, but it's getting close.

Change-Id: Iad01d1fc02c3cd238d73b9b9e02dbb4301cb6f2a

docs/tutorials/architecture.md

@@ -25,3 +25,8 @@
   alt="PresentationTimeline diagram"
   style="max-width: 100%">
 
+<img
+  src="../design/newdemo.gv.png"
+  alt="Demo page architecture"
+  style="max-width: 100%">
+

docs/tutorials/ui.md

@@ -79,8 +79,15 @@ function onUIErrorEvent(errorEvent) {
   // Handle UI error
 }
 
+function initFailed() {
+  // Handle the failure to load
+}
+
 // Listen to the custom shaka-ui-loaded event, to wait until the UI is loaded.
 document.addEventListener('shaka-ui-loaded', init);
+// Listen to the custom shaka-ui-load-failed event, in case Shaka Player fails
+// to load (e.g. due to lack of browser support).
+document.addEventListener('shaka-ui-load-failed, initFailed);
 
 ```