Add job failure reasons reference to troubleshooting docs
Adds a troubleshooting topic to doc/ci/jobs/job_troubleshooting.md explaining when GitLab automatically drops a stuck or timed-out job, and the threshold for each of the specific failure reasons introduced in !237556.
Scoped to the stuck and timeout family only, not a complete job failure reasons reference. Other failure reason categories (quota, bridge and downstream pipelines, deployments, and so on) don't have a time-based threshold to document here and need their own cause and fix explanations, so they're tracked as a separate follow-up instead of included in this table.
References
Edited by Lysanne Pinto