add devShell

flake.nix

···
       packages = forEachPkgs (pkgs: {
         default = pkgs.callPackage ./package.nix { };
       });
+
+      devShells = forEachPkgs (pkgs: {
+        default = pkgs.mkShell {
+          buildInputs = [ pkgs.zola ];
+        };
+      });
     };
 }