compose.yaml, setup-spoke.sh: scope syncthing mount to data directory only, move config/certs to named Docker volume, syncthing.sh: update add folder path example

spoke/compose.yaml

@@ -28,4 +28,8 @@ services:
       - "127.0.0.1:8384:8384"
       - "22000:22000"
     volumes:
-      - /home/armbian/st:/var/syncthing
+      - syncthing-config:/var/syncthing/config
+      - /home/armbian/st/data:/var/syncthing/data
+
+volumes:
+  syncthing-config: