default-config: Spawn waybar at startup

Make it a bit less of an empty screen.

resources/default-config.kdl

@@ -247,7 +247,9 @@ layout {
 // Note that running niri as a session supports xdg-desktop-autostart,
 // which may be more convenient to use.
 // See the binds section below for more spawn examples.
-// spawn-at-startup "alacritty" "-e" "fish"
+
+// This line starts waybar, a commonly used bar for Wayland compositors.
+spawn-at-startup "waybar"
 
 // Uncomment this line to ask the clients to omit their client-side decorations if possible.
 // If the client will specifically ask for CSD, the request will be honored.