docs: add physical connection map

[jmgilman]

Summary

• add an authoritative port-to-port map for all 18 installed cables
• record the two explicitly unconnected ports
• assign stable PHY connection identifiers without mixing in VLAN or interface configuration
• make the physical map canonical from the hardware and design pages
• add landing-page and site navigation

Verification

• validated 18 cables, 36 unique connected endpoints, and 2 unconnected ports
• built the documentation with MkDocs strict mode
• browser-checked the rendered connection table and cross-document links