[Testing] Application testing in Symfony is fun! Let's document the modern features

[wouterj]

This PR continues the efforts of restructuring the Testing docs:

• Focus more on Arrange, Act, Assert
• Advertise modern testing features (assertions). Somehow, we didn't mention any easy to use assertion in the main guide, but instead explained the complex dom crawler API.
• Reduce some "duplication". There is an astonishing number of ways to fill in and submit a form (we documented at least 4 different APIs in the main guide!). I've reduced this to two: a plain and short one, and a longer one for more complex form submissions.

Todo, preferably in a follow-up PR: combine the testing/dom_crawler and components/dom_crawler feature documentation into testing/dom_crawler.

cc @Nyholm

[javiereguiluz]

I like it! It's a nice simplification + reorganization. Thanks Wouter!

[javiereguiluz]

Thanks Wouter! This is now merged.

[wouterj]

Thank you @javiereguiluz. If you need help with the up merging, feel free to ping me :)

[javiereguiluz]

I've just finished upmerging. I think I did it right, but I'm not 100% sure, so maybe you could check the result. Thanks! I'll let you merge the other one (#15305) so you can easily spot the confilcts 🙏 Thanks!