Added landing pages for new samples

src/site/samples/notifications/index.markdown

+---
+layout: default
+title: Using the Notifications API
+live_example_url: example/index.html
+
+header:
+  css: ["/samples/samples.css"]
+---
+
+## {{ page.title }}
+
+A simple example showing use of the Notifications API. To find out more about
+this API, read the
+[Using the Notifications
+API](http://www.html5rocks.com/en/tutorials/notifications/quick) article by
+Ernest Delgado, originally published on HTML5Rocks.
+
+The Notifications API allows you to display notifications to the user for
+given events, both passively (new emails, tweets, or calendar events) and on
+user interactions.
+
+You can open the example in Dart Editor and run it by clicking `index.html`.
+Read the
+[source](https://github.com/dart-lang/dart-samples/tree/master/web/html5/notifications/quick).
+
+<iframe class="running-app-frame"
+        style="height:500px;width:100%;"
+        src="{{page.live_example_url}}">
+</iframe>
+
+See all [samples](/samples/).