nixos containers: disable NixOS manual in container config. (#75659)

This makes ~2.5x speed up of an empty container instantiate, hence reduces
rebuild time of system with many declarative containers.

Note that this doesn't affect production systems much, becaseu those most
likely already include `minimal.nix` profile.

nixos/modules/virtualisation/container-config.nix

@@ -10,6 +10,7 @@ with lib;
     nix.optimise.automatic = mkDefault false; # the store is host managed
     services.udisks2.enable = mkDefault false;
     powerManagement.enable = mkDefault false;
+    documentation.nixos.enable = mkDefault false;
 
     networking.useHostResolvConf = mkDefault true;