Sort subcommands by section

Sections are assigned while defining the list to make sure they are
still in sync.

Author: Julow

doc/dune

@@ -9,7 +9,8 @@
   interface
   ocamldoc_differences
   odoc_for_authors
-  parent_child_spec))
+  parent_child_spec
+  manpage))
 
 (rule
  (alias docgen)