Second iteration of documentation for writing end-to-end tests

What does this MR do?

Continuation of the Writing end-to-end tests step-by-step documentation.

This MR is related to this other one.

Note: If all is good, I think we could have it merged since it already provides value compared to the original documentation, and I can finish the Page Objects section in a third iteration.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content