docs: add Multi-AIPC PXE netboot deployment guide

[Stanley-PH]

Summary

Adds a runnable worked-example guide for deploying AUP Learning Cloud across three AIPCs via PXE netboot (installation/multi-node/multi-aipc-hardware-deployment), and wires it into the installation toctree and multi-node index.

The guide covers the full path end to end:

• Architecture overview + diagram (basic-example-pxe-architecture.svg)
• PXE controller playbook configuration (operator fills in site IPs/SSH keys; no real values in the guide)
• Manual K3s server install + publishing token/kubeconfig for netboot agents
• Agent netboot, hostname, local-disk persistence, and auto-join
• AMD GPU device plugin + ROCm labeller
• Shared NFS storage for notebook PVCs
• JupyterHub values + chart deploy, and end-to-end validation (including spawning a course notebook)

Notes

• Pairs with the pxe_controller Ansible role change in aup-learning-cloud (code PR).
• Section 4 documents the shipped mount-local-disk persistence (incl. an mkfs.ext4 safety note); "Out Of Scope" carries a <!-- --> marker for future config tutorials (Cloudflare/WARP/Zot/etc.).

Test plan

• Docs build (make html) renders the new page + toctree without warnings
• Architecture SVG displays and cross-links resolve

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