Add multi-site Caddy helpers and document usage

- add startup/shutdown scripts that render a Caddyfile from JSON config and run health checks

- add Python utilities and a sample sites.json for declarative multi-site configuration

- document the workflow and ignore generated Caddy state artifacts

- normalize double-quote style across challenge workflow controllers, nodes, and tests

docker/caddy/__init__.py

@@ -0,0 +1,11 @@
+"""Utilities for generating multi-site Caddy configurations."""
+
+from .render_caddy import SiteMetadata, render_sites, render_metadata_lines
+from .load_metadata import load_metadata
+
+__all__ = [
+    "SiteMetadata",
+    "render_sites",
+    "render_metadata_lines",
+    "load_metadata",
+]