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

.env.example

@@ -4,6 +4,14 @@
 # so they can be mounted into the container without env-var leakage. This
 # .env only holds non-secret tunables.
 
+# ---------------------------------------------------------------------------
+# Your root domain. This single value flows into authelia/configuration.yml
+# (via Authelia's {{ env "DOMAIN" }} template substitution) and into
+# caddy/Caddyfile (via Caddy's {env.DOMAIN} substitution).
+# No manual find-and-replace needed -- just set this.
+# ---------------------------------------------------------------------------
+DOMAIN=example.com
+
 # Pin your image versions. Bump to current stable when you upgrade --
 # check https://github.com/authelia/authelia/releases and
 # https://github.com/crazy-max/docker-fail2ban/releases.