docs: build docs using mdBook

docs/turn.md

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+# Setting up TURN/STURN
+
+## General instructions
+
+* It is assumed you have a [Coturn server](https://github.com/coturn/coturn) up and running. See [Synapse reference implementation](https://github.com/matrix-org/synapse/blob/develop/docs/turn-howto.md).
+
+## Edit/Add a few settings to your existing conduit.toml
+
+```
+# Refer to your Coturn settings. 
+# `your.turn.url` has to match the REALM setting of your Coturn as well as `transport`.
+turn_uris = ["turn:your.turn.url?transport=udp", "turn:your.turn.url?transport=tcp"]
+
+# static-auth-secret of your turnserver
+turn_secret = "ADD SECRET HERE"
+
+# If you have your TURN server configured to use a username and password
+# you can provide these information too. In this case comment out `turn_secret above`!
+#turn_username = ""
+#turn_password = ""
+```
+
+## Apply settings
+
+Restart Conduit.