nixos manual: cleanup generation

nixos/doc/manual/configuration/configuration.xml

···
 <xi:include href="networking.xml" />
 <xi:include href="linux-kernel.xml" />
 
-<xi:include href="emacs.xml" />
 <xi:include href="modules.xml" xpointer="xpointer(//section[@id='modules']/*)" />
 
 <!-- Apache; libvirtd virtualisation -->

nixos/doc/manual/default.nix

···
     ''
       cp -prd $sources/* . # */
       chmod -R u+w .
-      cp ${../../modules/services/databases/postgresql.xml} configuration/postgresql.xml
-      cp ${../../modules/services/misc/gitlab.xml} configuration/gitlab.xml
-      cp ${../../modules/services/misc/taskserver/doc.xml} configuration/taskserver.xml
-      cp ${../../modules/security/acme.xml} configuration/acme.xml
-      cp ${../../modules/i18n/input-method/default.xml} configuration/input-methods.xml
-      cp ${../../modules/services/editors/emacs.xml} configuration/emacs.xml
       ln -s ${modulesDoc} configuration/modules.xml
       ln -s ${optionsDocBook} options-db.xml
       echo "${version}" > version

nixos/modules/services/editors/emacs.nix

···
       EDITOR = mkOverride 900 "${editorScript}/bin/emacseditor";
     } else {};
   };
+
+  meta.doc = ./emacs.xml;
 }