Add job failure reasons reference to troubleshooting docs

What does this MR do and why?

Adds a job failure reasons reference table to doc/ci/jobs/job_troubleshooting.md to help users understand why their jobs failed.

It includes the new failure reasons introduced in !237556, along with the existing ones for retry:when.

References

• Clarify and differentiate stuck_or_timeout job ... (#595752 - closed)
• Draft: Clarify stuck_or_timeout_failure retry r... (!237556)
• Add troubleshooting docs for downstream pipelin... (!237748 - merged)