Document the new CI_DEBUG_TRACE variable

What does this MR do?

Add documentation for a feature that will be introduced in GitLab Runner v1.7

Why was this MR needed?

Documentation!

Are there points in the code the reviewer needs to double check?

It contains dire warnings about the ability of the debug trace to leak all your secrets. Are these dire enough? Are the avoidance instructions clear enough?

Ideally, GitLab would detect the presence of a debug trace and do all this magically. Perhaps in a later release.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Related to https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/339