nixos manual: move chapter on grsecurity to auto-generated module docs

nixos/doc/manual/configuration/configuration.xml

···
 <xi:include href="x-windows.xml" />
 <xi:include href="networking.xml" />
 <xi:include href="linux-kernel.xml" />
-<xi:include href="grsecurity.xml" />
 
 <xi:include href="emacs.xml" />
 <xi:include href="modules.xml" xpointer="xpointer(//section[@id='modules']/*)" />

nixos/modules/security/grsecurity.nix

···
 in
 
 {
+  meta = {
+    maintainers = with maintainers; [ joachifm ];
+    doc = ./grsecurity.xml;
+  };
+
   options.security.grsecurity = {
 
     enable = mkEnableOption "grsecurity/PaX";