nixos/clan: add option for opting out of Clan defaults

Also replace `documentation.nixos.enable = false` with
`documentation.doc.enable` to opt out of only `nixos-help` and the HTML
manual but leave `man configuration.nix`.

nixosModules/clanCore/networking.nix

@@ -91,7 +91,7 @@
       ]
     )
   ];
-  config = {
+  config = lib.mkIf config.clan.core.setDefaults {
     # conflicts with systemd-resolved
     networking.useHostResolvConf = false;