[doc] add app doc to serve doc

'nix run .#doc' will open documentaion in browser and update on
filechange

outputs.nix

@@ -37,6 +37,16 @@ in (utils.lib.eachSystem (builtins.filter filter_system utils.lib.defaultSystems
         '';
       };
     };
+
+    apps = {
+      docs = {
+        type = "app";
+        program = builtins.toString (pkgs.writeScript "docs" ''
+          ${pkgs.mdbook}/bin/mdbook serve --open ./doc
+        '');
+      };
+    };
+
   })) // rec {
     nixosConfigurations = import ./machines/configuration.nix (inputs // {
       inherit inputs;