Testing Docs: Add links to jump to sections (#31311)

Added Playwright to the top of the page and links to help readers jump to sections. 

## 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
- [x] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [x] Make sure the linting passes by running `yarn lint`

docs/testing.md

@@ -13,7 +13,7 @@ description: Learn how to set up Next.js with three commonly used testing tools
   </ul>
 </details>
 
-Learn how to set up Next.js with three commonly used testing tools: [Cypress](https://www.cypress.io/blog/2021/04/06/cypress-component-testing-react/), [Jest](https://jestjs.io/docs/tutorial-react), and [React Testing Library](https://testing-library.com/docs/react-testing-library/intro/).
+Learn how to set up Next.js with commonly used testing tools: [Cypress](https://nextjs.org/docs/testing#cypress), [Playwright](https://nextjs.org/docs/testing#playwright), and [Jest with React Testing Library](https://nextjs.org/docs/testing#jest-and-react-testing-library).
 
 ## Cypress