docs: add tenant lifecycle cleanup operations guide

[martinraumann]

Summary

Adds an Operations guide page for tenant lifecycle cleanup after instance release. The page covers the tenant-facing release path, cleanup state flow, sanitization steps, trust and platform checks, return-to-pool criteria, and stuck-cleanup triage.

Details

• Uses nicocli as the primary operator-facing workflow for instance deletion and status history.
• Uses carbide-admin-cli only for Core/site cleanup inspection that is not exposed through the REST workflow.
• Documents cleanup states from release through Ready.
• Adds concrete checks for cleanup progress, metrics, sanitization results, measured boot, health reports, and manual site-policy procedures.
• Adds the page to the Operations navigation.

Validation

• Ran git diff --check.
• Reviewed commands and status-history behavior against the current docs, OpenAPI spec, and REST CLI command generation.

Notes

This is a draft for developer and technical-writer review before publishing.

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