feat: add files module, text helper, and update readme generation

modules/flake-parts/files.nix

@@ -0,0 +1,11 @@
+{ inputs, ... }:
+{
+  imports = [ inputs.files.flakeModules.default ];
+  perSystem = psArgs: {
+    make-shells.default.packages = [ psArgs.config.files.writer.drv ];
+  };
+  text.readme.parts.generated-files = ''
+    This flake uses the [files flake-parts module](https://flake.parts/options/files.html) to generate documentation.
+    The list of generated files are:
+  '';
+}