deps: Add websocketpp as vendored code

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

deps/websocketpp/docs/tutorials.dox

@@ -0,0 +1,10 @@
+/** \page tutorials Tutorials
+
+These tutorials are works in progress, some are more complete than others.
+
+- \subpage md_tutorials_utility_client_utility_client
+- \subpage md_tutorials_utility_server_utility_server
+- \subpage md_tutorials_broadcast_tutorial_broadcast_tutorial
+- \subpage md_tutorials_chat_tutorial_chat_tutorial
+
+*/