docs: update react-testing-library section

Created by: john-james-gh

What:

Updates the section on react-testing-library where the docs imply an installation of both @testing-library/react and @testing-library/jest-dom is necessary.

Why:

react-scripts version 3.3.0 adds @testing-library/react and @testing-library/jest-dom as default testing library/utility which eliminates the need for installation and/or configuration in src/setupTests.js

Source: https://github.com/facebook/create-react-app/pull/7881 and https://github.com/facebook/create-react-app/releases?after=react-scripts%403.3.1

How:

I added a similar "Note: " block that is used throughout the docs.

Screenshots:

Before

After