Fix error message bug, improve docs and comments, expand test coverage

[Copilot]

The repo had a copy-paste bug in an error message, sparse documentation, and limited test coverage.

Bug

• UnmarshalCaddyfile: "API token already set" → "endpoint already set" when endpoint is supplied both inline and as a subdirective

Documentation

• Added package-level doc comment linking the Caddy module to its libdns backing
• Provision: comment now describes what it actually does (Caddy placeholder expansion)
• UnmarshalCaddyfile: doc block shows all three valid syntax forms; notes credentials are optional
• README: linked upstream ACMEProxy project, showed both Caddyfile endpoint forms, noted optional credentials, added {env.VAR} placeholder mention

Tests

• Fixed mislabeled t.Errorf in test loop (Provider.CaddyModule() → Provider.UnmarshalCaddyfile())
• Added three UnmarshalCaddyfile cases: endpoint-only via inline arg + empty block, via subdirective, and bare inline arg with no block
• Added TestProvider_CaddyModule covering module ID, non-nil New, and correct return type