fix TURN/STURN

* remove docker-compose-coturn.yml and replace it with and example-file,
because there are user-data inside, which should not be on git
* add a whole rtc_config_example-coturn.json to be copied to
  rtc_config.json and only the domain <DOMAIN> should be changed.
* modified the documentation
* modified the .gitignore to ignore the files with user-data

rtc_config_example-coturn.json

@@ -0,0 +1,21 @@
+{
+  "sdpSemantics": "unified-plan",
+  "iceServers": [
+    {
+        "urls": "stun:<DOMAIN>:3478"
+    },
+    {
+        "urls": "stuns:<DOMAIN>:5349"
+    },
+    {
+        "urls": "turn:<DOMAIN>:3478",
+        "username": "user",
+        "credential": "password"
+    },
+    {
+        "urls": "turns:<DOMAIN>:5349",
+        "username": "user",
+        "credential": "password"
+    }
+  ]
+}