manual.md: add labnag(1)

Author: johanmalm

src/manual.md

@@ -7,6 +7,7 @@
 [labwc-theme(5)](labwc-theme.5.html)  
 [labwc-menu(5)](labwc-menu.5.html)  
 [labwc-actions(5)](labwc-actions.5.html)  
+[labnag(1)](labnag.1.html)
 
 ## Diagrams