Document workspace debug mode for diagnosing startup failures

[snecklifter]

Problem

The workspace debug mode feature (controller.devfile.io/debug-start annotation) is completely undocumented in che-docs. Users have no guidance on:

• What debug mode does (keeps failed containers running, traps postStart failures)
• How to enable it via the dashboard UI
• How to inspect logs (/tmp/poststart-stderr.txt, /tmp/poststart-stdout.txt)
• That manually patching the annotation is overwritten by the dashboard

Proposed Solution

Add a new PROCEDURE page debugging-a-workspace.adoc to the end-user guide troubleshooting section covering:

1. What debug mode does and when to use it
2. Starting a stopped workspace in debug mode ("Open in Debug mode")
3. Restarting a running workspace in debug mode ("Restart in Debug mode" — requires Add 'Restart in Debug mode' workspace action to dashboard #23863)
4. Inspecting postStart logs via kubectl exec
5. A warning that manual annotation patching is overwritten by the dashboard

Related

• Dashboard fix: Add 'Restart in Debug mode' workspace action to dashboard #23863 — adds the "Restart in Debug mode" action referenced in the docs
• Operator: The annotation behavior is documented in the devworkspace-operator additional-configuration.adoc

[snecklifter]

Docs PR: eclipse-che/che-docs#3111
Related dashboard issue: #23863
Related dashboard PR: eclipse-che/che-dashboard#1593