docs(networking): finalize core network design

[jmgilman]

Summary

• accept and promote the Lab v2 core network design
• add the canonical address, VLAN, DHCP, interface, and switch-port reference
• record the MS-02 management/OOB split and repurpose the UM760 as sandbox01
• retire PXE/Tinkerbell, PowerDNS, artifact serving, bootstrap-k0s, and BGP from the gateway design

Verification

• mise exec -- moon run docs:build --summary minimal
• git diff --check
• read-only live checks of VP6630 link state and interface speeds
• official MINISFORUM MS-02 Ultra rear-panel and NIC specifications