Add existing Kubernetes deployment runbook

[MioYuuIH]

Summary

• add a command-first deployment runbook for installing AUP Learning Cloud on an existing Kubernetes cluster
• document checksum-verified AMD GPU setup, storage and values preparation, Helm deployment, acceptance checks, and troubleshooting
• refocus the Multi-Node K3s and PXE guides on cluster provisioning and hand off application deployment to the canonical Kubernetes runbook
• update installation navigation and deployment-path guidance

Validation

• Sphinx HTML build passes with only the three existing cross-reference warnings
• linkcheck reports zero internal or target-page failures; three unrelated external links retain their existing TLS/403 failures
• all 27 Bash command blocks pass syntax parsing after documented placeholder substitution
• pinned AMD manifests reproduce the documented SHA256 checksums
• desktop and 390x844 mobile visual QA pass with zero page-level horizontal overflow
• two independent visual reviewers report PASS with no blocking findings