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

syncthing.sh

@@ -222,7 +222,7 @@ else:
 
 add_folder() {
     header "Add Folder"
-    read -rp "Folder path on this device (e.g. /var/syncthing/docs): " FOLDER_PATH
+    read -rp "Folder path on this device (e.g. /var/syncthing/data/books): " FOLDER_PATH
     [ -n "$FOLDER_PATH" ] || die "Path cannot be empty."
     read -rp "Folder label (human-readable name): " FOLDER_LABEL
     [ -n "$FOLDER_LABEL" ] || die "Label cannot be empty."