docs: add initial core network design

[jmgilman]

Summary

• add the initial Lab v2 core-network draft
• record the VyOS Layer 3 and MikroTik Layer 2 responsibility split in ADR-0001
• add a hardware reference that separates legacy evidence from physical verification
• add site navigation for decisions, designs, and reference material

Verification

• built the site with MkDocs strict mode through moon run docs:build
• reviewed prose for unsupported claims and language-style trigger phrases
• browser-verified the home, design, ADR, and hardware pages and their navigation