Document the :repository and :custom_repo traits

What does this MR do?

Adds a smattering of information about the repository-creating traits to the testing best practices. We should steer people away from using gitlab-test where possible, as it's essentially a fixture.

What are the relevant issue numbers?

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

Closes #51366 (closed)