Improve end-to-end testing quick start guide

What does this MR do?

This MR improves the end-to-end testing quick start guide in the 'Are end-to-end tests needed?' section, as a way of helping with the new responsibilities of Software Engineers in Test.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content