Unbust doc build

Argh. My guess is that the problem is in Pod::To::BigPage as it
expects a non-empty subtitle and starts crying if it's empty. That
should be fixed properly, but for now I just want to let the docs
website rebuild. Also, we should ot have these categories as table
rows, but I believe that the work for that is in progress.

Author: AlexDaniel

manage-page-order.p6

@@ -401,6 +401,7 @@ sub write-group-page($group-char, $title, $page-order) {
     $fh.print: qq:to/HERE/;
         =begin pod :page-order<{$page-order}>
         =TITLE $title
+        =SUBTITLE ~
         =end pod
         HERE