Document safe Countdown job cancellation and cleanup

[vimalcha]

Summary

• Add a Countdown-specific, one-job-at-a-time cancellation and cleanup guide with project and job confirmations.
• Link the guide from the demo and post-training docs; clarify the grader deployment step and remove the redundant display-region setting.
• Add offline cancellation-contract tests to the managed documentation workflow.

Validation

• Root unittest suite passed (7 tests).
• CI-selected managed offline pytest suite passed (105 tests).
• No notebook cells or Azure/Foundry operations executed.