Improve the static files example. (#317)

* Improve the static files example.

* Replace the broken websocket example with a simple static HTML page.
* Show how to serve a second directory which is located inside the main
  directory but has file listing enabled.
* Demonstrate that a hierarchy of static files can be served by placing the
  JS and CSS files in their respective subfolders.
* Document how to use the example in the README.

* Improve formatting.

static_index/static/root/css/example.css

@@ -0,0 +1,11 @@
+body {
+    text-align: center;
+}
+
+h1 {
+    font-family: sans-serif;
+}
+
+img {
+    transition: .5s ease-in-out;
+}