Add Playwright example

[mxschmitt]

Playwright can be used for writing end-to-end tests across Chromium, Firefox, and WebKit which is supported on all the platforms.

I'll follow up by adding it to the testing.md section: #29832

Documentation / Examples

• Make sure the linting passes

[mxschmitt]

We should not forget about this one, otherwise the example in the docs is not working.

cc @timneutkens
cc @molebox

Thank you!

[benrobot]

I don't know how long these PR usually take to get reviewed and merged but I couldn't wait to start using this example so I went ahead and ran the following command instead:

npx create-next-app --example https://github.com/mxschmitt/next.js/tree/add-playwright/examples/with-playwright with-playwright-app

[benrobot]

I believe this pull request will fix the broken link currently live at https://nextjs.org/docs/testing