Added landing pages for new samples

src/site/samples/webaudio/index.markdown

+---
+layout: default
+title: Getting Started with the Web Audio API
+live_example_url: example/index.html
+
+header:
+  css: ["/samples/samples.css"]
+---
+
+## {{ page.title }}
+
+A port of an example used in the
+[Getting Started with Web Audio API](http://www.html5rocks.com/en/tutorials/webaudio/intro/)
+article by Boris Smus, originally published on HTML5Rocks.
+
+You can use the Web Audio API for processing and synthesizing audio in web
+applications. The API lets you use functionality found in modern game audio
+engines and mixing, processing, and filtering tasks found in modern desktop
+audio production applications.
+
+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/webaudio/intro).
+
+<iframe class="running-app-frame"
+        style="height:500px;width:100%;"
+        src="{{page.live_example_url}}">
+</iframe>
+
+See all [samples](/samples/).