[Documentation] Add a tutorial how to create tests using the Langium testing API

[Lotes]

• parseHelper
• validationHelper
• look for more under langium/test

[spoenemann]

I would even propose to create a chapter in the main documentation giving an overview of our testing utilities.

[acherm]

Yes, I strongly support to have such a documentation. I have written something: https://dev.to/diverse_research/testing-your-dsls-in-langium-gp9 with a working setup. But I have suspected that I miss some API understanding of langium/test or some best practices. And the answer here: https://dev.to/lotes/comment/2aiog confirmed my intuition ;)