docs: add 3 Node Mini-Cluster Example (PXE netboot) guide

[KerwinTsaiii]

Summary

• Adds an end-to-end Installation guide: 3 Node Mini-Cluster Example, covering how to build a small K3s cluster by PXE-netbooting diskless AMD Ryzen AI agents from one service machine and deploying AUP Learning Cloud on top.
• One service machine (AIPC 1) hosts the PXE controller, NFS rootfs, Apache K3s credential endpoint, the single-node K3s server, and notebook NFS storage; the agents are diskless and auto-join by netboot.
• Reuses the existing k3s_server role via inventory for the server install; documents publishing the K3s token/kubeconfig, GPU device plugin, NFS provisioner, JupyterHub values, and deploy + end-to-end validation.
• Includes a header banner, the PXE architecture diagram, VM-example netboot screenshots, the Hub URL + home-page screenshot, and a Scope and Limitations section noting the single-host / single-point-of-failure trade-offs.
• Wires the new page into the Installation toctree in source/index.rst.

Notes

• The companion pxe_controller Ansible role + playbook live on a separate branch off develop (feat/pxe-controller); this PR is docs-only.

Test plan

• make html builds successfully (only pre-existing unrelated xref warnings).
• New page renders, images load, and it appears in the Installation toctree.
• Reviewer visual check of the rendered page.

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