nixos/make-options-doc: turn relatedPackages into links

link to search.nixos.org instead of pulling package metadata out of pkgs. this
lets us cache docs of a few more modules and provides easier access to package
info from the HTML manual, but makes the manpage slightly less useful since
package description are no longer rendered.

nixos/modules/services/web-apps/nextcloud.nix

@@ -932,6 +932,4 @@ in {
   ]);
 
   meta.doc = ./nextcloud.xml;
-  # uses relatedPackages
-  meta.buildDocsInSandbox = false;
 }