Third iteration on writing end to end tests doc

What does this MR do?

Finishes the documentation that will make it easier for other engineers to contribute with end-to-end tests for GitLab.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/62158.

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