Reference core Testing Library documentation/principles in documentation

[jrolfs]

I thought it was reasonable to assume that these concepts were implied, but I think we should be more explicit. For example, here is how it's referenced in Preact Testing Library: https://testing-library.com/docs/preact-testing-library/intro#the-problem.

Some examples of issues I hope this would provide clarification for:

#517
#518
#511
#521
#519