docs: add sections and fix typos/errors

few side refactors of this:
- no more `dns.domain`, it all must rely on `fqdn`, prevents
  inconsistencies.
- also added an specific host `tuxcord-acmetest` that uses the key zone
  for `nix.tuxcord.net` to test certificate pulling.

nixos/openssh.nix

@@ -10,14 +10,6 @@
       PermitRootLogin = "no";
     };
   };
-<<<<<<< HEAD
 
   networking.firewall.allowedTCPPorts = [ 22 ];
-||||||| parent of 1c2f11d (lib/ssh: add more ssh keys)
-
-  users.users.root.openssh.authorizedKeys.keys = builtins.attrValues {
-    inherit (import "${self}/lib/ssh/keys.nix") error javalsai;
-  };
-=======
->>>>>>> 1c2f11d (lib/ssh: add more ssh keys)
 }