📝(livekit) add a room-composite Egress request example

This .json file will be useful when calling the LiveKit CLI command
"start-egress", which takes a "--request" params as a path to a json file.

docker/livekit/egress-example/room-composite-file.json

@@ -0,0 +1,9 @@
+{
+  "room_name": "your-room-name",
+  "layout": "speaker-dark",
+  "file_outputs": [
+    {
+      "filepath": "/out/"
+    }
+  ]
+}