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>

Dockerfile

@@ -15,7 +15,7 @@ COPY . .
 RUN mkdir -p /app/data
 
 # Accept build arguments
-ARG SITE_URL=https://nlcc.rydertech.us
+ARG SITE_URL=https://church.example.com
 ARG EMAIL_HOST=smtp.example.com
 ARG EMAIL_PORT=587
 ARG EMAIL_USER=noreply@example.com