docs: explain deploy test custom scripts (#90966)
## Summary
- document how to use custom deploy, logs, and cleanup scripts with the
deploy test workflow
- describe the expected stdout/log marker contract for custom scripts
- keep the examples generic to adapter implementations
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Co-authored-by: Vercel <vercel[bot]@users.noreply.github.com>
Co-authored-by: Tim Neutkens <tim@timneutkens.nl>
