Create u_e-webmail-site.md

docs/u_e-webmail-site.md

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+To create a subdomain `webmail.example.org` and redirect it to SOGo, you need to create a new Nginx site. Take care of "CHANGE_TO_MAILCOW_HOSTNAME"!
+
+**nano data/conf/nginx/webmail.conf**
+``` hl_lines="17"
+server {
+  ssl_certificate /etc/ssl/mail/cert.pem;
+  ssl_certificate_key /etc/ssl/mail/key.pem;
+  index index.php index.html;
+  client_max_body_size 0;
+
+  include /etc/nginx/conf.d/listen_plain.active;
+  include /etc/nginx/conf.d/listen_ssl.active;
+  server_name webmail.example.org;
+
+  location ^~ /.well-known/acme-challenge/ {
+    allow all;
+    default_type "text/plain";
+  }
+
+  location / {
+    return 301 https://CHANGE_TO_MAILCOW_HOSTNAME/SOGo;
+  }
+}
+```
+
+Save and restart Nginx: `docker-compose restart nginx-mailcow`.
+
+Now open `mailcow.conf` and find `ADDITIONAL_SAN`.
+Add `webmail.example.org` to this array, don't use quotes!
+
+```
+ADDITIONAL_SAN=webmail.example.org
+```
+
+Run `docker-compose up -d`. See "acme-mailcow" and "nginx-mailcow" logs if anything fails.