Added landing pages for new samples

src/site/samples/websockets/index.markdown

+---
+layout: default
+title: A Basic WebSockets Example
+live_example_url: example/index.html
+
+header:
+  css: ["/samples/samples.css"]
+---
+
+## {{ page.title }}
+
+A simple example of using WebSockets for communication.
+
+You can learn about the basics of WebSockets by reading the
+[Introducing WebSockets: Bringing Sockets to the Web](http://www.html5rocks.com/en/tutorials/file/filesystem/)
+article on HTML5Rocks.
+
+WebSocket is a protocol providing full-duplex communications channels over a
+single TCP connection.
+
+This example opens a WebSocket connection to an echo server
+(`ws://echo/websocket.org`), sends data to the server, and then displays the
+response on the web page.
+
+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/websockets/basics).
+
+<iframe class="running-app-frame"
+        style="height:500px;width:100%;"
+        src="{{page.live_example_url}}">
+</iframe>
+
+See all [samples](/samples/).