Comprehensive documentation overhaul for Charon features:
Rewrite features.md as marketing overview (87% reduction)
Create comprehensive dns-challenge.md for new DNS feature
Expand 18 feature stub pages into complete documentation:
SSL certificates, CrowdSec, WAF, ACLs, rate limiting
Security headers, proxy headers, web UI, Docker integration
Caddyfile import, logs, WebSocket, backup/restore
Live reload, localization, API, UI themes, supply chain security
Update README.md with DNS Challenge in Top Features
Total: ~2,000+ lines of new user-facing documentation
Refs: #21, #461