Add "Best practices" page

Quoting @sarahdayan:

Additionally, it could be nice to have a Best practices section. Kent often writes blog posts, tweets, or GitHub replies on these topics that get scattered across the Internet, and it's tough to find what you need. Having a canonical place for these would be helpful.

For the time being and to keep it attainable, it could be a list of posts/resources that the Testing Library shares as The Proper Way™️ of using the library.

We should check if content from Learning (https://testing-library.com/docs/learning) is still valid and worth mentioning nowadays.

Start by reviewing the existing Learning page at https://testing-library.com/docs/learning and determine which material remains valid. Add a Best practices page containing the approved Testing Library posts and resources; it is done when the page provides a current, canonical list and links to worthwhile Learning content.