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/js/example.js

@@ -0,0 +1,7 @@
+jQuery(document).ready(function () {
+    let rotation = 0;
+    jQuery("img").click(function () {
+        rotation += 360;
+        jQuery("img").css({'transform': 'rotate(' + rotation + 'deg)'});
+    });
+});