docs: add workspace debug mode documentation

[snecklifter]

Summary

• Adds a new procedure page debugging-a-workspace.adoc documenting how to use debug mode for diagnosing workspace startup failures
• Covers the dashboard UI actions ("Open in Debug mode" for stopped workspaces, "Restart in Debug mode" for running workspaces)
• Documents how to inspect postStart logs at /tmp/poststart-stderr.txt and /tmp/poststart-stdout.txt
• Includes a warning that manually patching the controller.devfile.io/debug-start annotation is overwritten by the dashboard
• Adds the page to the troubleshooting navigation and assembly page

Test plan

• Verify AsciiDoc renders correctly in Antora build
• Verify all cross-references resolve (nav.adoc, troubleshooting-che.adoc)
• Verify attribute substitutions ({prod-short}, {orch-cli}, etc.) work correctly

Related

• Fixes Document workspace debug mode for diagnosing startup failures che#23864
• Dashboard PR: feat: add 'Restart in Debug mode' workspace action che-dashboard#1593 (adds the "Restart in Debug mode" action referenced in this doc)
• Related issue: Add 'Restart in Debug mode' workspace action to dashboard che#23863 (dashboard fix)

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