Document better the process around quarantined tests

What does this MR do?

This better documents the process around quarantined tests:

• When to quarantine tests and what are the prerequisite
• What to do with quarantined tests
• Quarantined tests on the CI

What are the relevant issue numbers?

Follow-up of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24369#note_131832106.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated via this MR
• [-] Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer