doc: Overhaul: Debug logging section of GitLab CI/CD environment variables

Problem to solve

Debug logging section in the GitLab CI/CD environment variables in GitLab CI/CD docs is too odd as it has been partially updated when feature(s) and/or deprecation(s) are introduced. The document was initially introduced in GitLab 8.13 so an overhaul should be considered.

Further details

On #55706 (closed), some irregular is fixed, but there are many odds to be fixed.

Proposal

As-is debug logging can be introduced with the results from the latest pipeline on https://gitlab.com/gitlab-examples/ci-debug-trace.