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/nix-settings.nix

@@ -1,7 +1,7 @@
-{ lib, ... }:
+{ lib, config, ... }:
 # Taken from:
 # https://github.com/nix-community/srvos/blob/main/nixos/common/nix.nix
-{
+lib.mkIf config.clan.core.setDefaults {
   # Fallback quickly if substituters are not available.
   nix.settings.connect-timeout = 5;