Include app directory in Tailwind config (#42407)

Updated with-tailwindcss example to include new _app_ directory according to [Tailwind docs](https://tailwindcss.com/docs/guides/nextjs)

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

examples/with-tailwindcss/tailwind.config.js

@@ -3,6 +3,7 @@ module.exports = {
   content: [
     './pages/**/*.{js,ts,jsx,tsx}',
     './components/**/*.{js,ts,jsx,tsx}',
+    './app/**/*.{js,ts,jsx,tsx}',
   ],
   theme: {
     extend: {},