docs: add Configuration examples section to README

[ryans-posthog]

Summary

• Adds a new ## Configuration examples section to README.md between Quick start and Overview.
• Examples cover: zero-config CLI one-liners, a minimal Caddyfile static file server, a reverse-proxy Caddyfile with automatic HTTPS, a multi-site Caddyfile with path routing, an equivalent JSON config, and admin API interaction via curl.
• Adds a matching Menu entry at the top of the README.

The existing Quick start section only linked out to caddyserver.com — a reader cloning the repo had no runnable example inline. This PR fills that gap without duplicating the upstream reference docs.

Test plan

• Render README.md on GitHub and confirm the new Menu entry anchors to the Configuration examples section
• Confirm fenced code blocks render with caddyfile, bash, and json highlighting
• Spot-check each example against a locally built caddy binary:
  • caddy file-server --listen :2015
  • caddy reverse-proxy --from :8080 --to :9000
  • caddy run against each Caddyfile example
  • curl localhost:2019/config/ against a running instance