feat: drive domain from DOMAIN env var -- no manual find-and-replace

Previously every example.com had to be found and replaced manually.
Now a single DOMAIN=yourdomain.com in .env propagates everywhere:

- .env.example: add DOMAIN=example.com with explanation
- docker-compose.yml: pass DOMAIN into authelia container environment
- authelia/configuration.yml: use {{ env "DOMAIN" }} in totp.issuer,
  access_control.rules, and all four session.cookies[] fields
  (Authelia 4.38+ Go template substitution)
- caddy/Caddyfile: use {env.DOMAIN} in all site block addresses
  (Caddy native env substitution); update header comment explaining
  how to set DOMAIN for system vs dockerized Caddy
- README.md: update step 3 to explain DOMAIN is the only change needed;
  update step 4 to say just uncomment the right rule; update Caddy
  wiring section with DOMAIN env var instructions for both Caddy modes

https://claude.ai/code/session_012eTokAaGiZo7aGt1T2W9BC

docker-compose.yml

@@ -34,6 +34,9 @@ services:
       - AUTHELIA_SESSION_SECRET_FILE=/secrets/SESSION_SECRET
       - AUTHELIA_STORAGE_ENCRYPTION_KEY_FILE=/secrets/STORAGE_ENCRYPTION_KEY
       - TZ=${TZ:-UTC}
+      # Passed through so authelia/configuration.yml can use {{ env "DOMAIN" }}
+      # instead of hardcoding your domain in the config file.
+      - DOMAIN=${DOMAIN}
     volumes:
       - ./authelia:/config
       - ./authelia/secrets:/secrets:ro