Merge branch 'stateVersion' into 'master'

Introduce stateVersion concept

See merge request simple-nixos-mailserver/nixos-mailserver!401

docs/setup-guide.rst

@@ -72,6 +72,7 @@ common ones.
 
      mailserver = {
        enable = true;
+       stateVersion = 1;
        fqdn = "mail.example.com";
        domains = [ "example.com" ];