Improved docs for testing

[enkelmedia]

Describe the bug
I've been working on a PR and trying to setup tests for it but it kind of hard to understand:

1. What kind of tests should be done (unit, e2e, both?).
2. How to create e2e test?
3. How to create unit test (only testing code, if possible?)
4. How can I run the tests?
5. How to run individual test?

May main objective for now is to create a unit test for a function that is parsing text.

Maybe someone can make a file similar to this: https://github.com/umbraco/Umbraco.CMS.Backoffice/blob/main/docs/authentication.md so that it'll be easier to contribute in a way that includes tests as well.