add demo script using websockets and asyncio

docs/echo.html

@@ -3,7 +3,7 @@
 <div id="data"></div>
 <script src="https://code.jquery.com/jquery-3.6.3.min.js"></script>
 <script>
-    let socket = new WebSocket("ws://localhost:5000/echo");
+    let socket = new WebSocket("ws://localhost:5000/");
     socket.onopen = function(e) {
       console.log("connection established");
       socket.send("Hello World");