add guidance on how to write a good acceptance test to extend the playwright test suite

[enekofb]

As part of the validation #3711
We raised the first PR with some suggestions weaveworks/playwright-tests#16

We think that would be useful, if we want a developer to add acceptance tests, to set some guidance on doing it the right way.

This ticket to achieve that

AC

• Exists guidance for how to write a good acceptance test to extend the playwright test suite

Notes:

• Ideally is formed by some basic principles, references and examples to get started.