docs(proxy): add DNS challenge documentation for Traefik & Caddy

[Cinzya]

Summary

• Adds a DNS challenge configuration guide for Traefik under knowledge-base/proxy/traefik/dns-challenge.md, covering Cloudflare, Hetzner, Hostinger, and Route53 with full annotated compose configs
• Adds a DNS challenge configuration guide for Caddy under knowledge-base/proxy/caddy/dns-challenge.md, covering the same providers — includes an inline dockerfile_inline build step since Caddy DNS modules must be compiled into the binary
• Both pages are linked from their respective nav sections in config.mts

Test plan

• Verify that the outlined steps for the DNS challenge work on both Traefik and Caddy
• Verify all code block annotations (++, --, focus) render correctly in dev server
• Confirm tab switching works across all provider tabs in both guides
• Check that internal links (wildcard certs, Lego provider list, caddy-dns org) resolve correctly
• Verify nav entries appear in the correct sidebar sections under Traefik and Caddy