chore: Replace specific domain with generic example URL

Changes:
- docker-compose.yml: https://nlcc.rydertech.us → https://church.example.com
- Dockerfile: https://nlcc.rydertech.us → https://church.example.com
- README.md: https://nlcc.rydertech.us → https://church.example.com

Rationale:
- Makes the repository more generic and reusable
- Users won't be confused by site-specific URLs
- Follows standard documentation practices using example.com
- Clearer that SITE_URL needs to be customized for each deployment

The example.com domain is reserved by IANA for documentation purposes,
making it the perfect choice for configuration templates.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docker-compose.yml

@@ -8,7 +8,7 @@ services:
       - nlcc-data:/app/data
     environment:
       - NODE_ENV=production
-      - SITE_URL=https://nlcc.rydertech.us
+      - SITE_URL=https://church.example.com
       # Optional: Customize admin username (default: "admin")
       # - ADMIN_USERNAME=admin
       # Optional: Set custom admin password (otherwise auto-generated)