Improve the 'Testing levels' documentation

What does this MR do?

This tries to address https://gitlab.com/gitlab-org/quality/team-tasks/issues/24 by (hopefully) improving the Testing levels documentation.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/quality/team-tasks/issues/24.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated
• [-] Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides