deps: Add websocketpp as vendored code

Make it easier to apply patches as official releases are not openssl4
compatible, yet.

deps/websocketpp/tutorials/chat_tutorial/chat_tutorial.md

@@ -0,0 +1,13 @@
+Chat Tutorial
+=============
+
+Goals of this tutorial:
+- Implement a realtime websocket chat server
+
+
+Server
+- Nicknames
+- Channels
+- Subprotocol
+- Origin restrictions
+- HTTP statistics page