docs: add how-to to setup roundcube

docs/add-roundcube.rst

@@ -0,0 +1,32 @@
+Add Roundube, a webmail
+=======================
+
+The NixOS module for roundcube nearly works out of the box with SNM. By
+default, it sets up a nginx virtual host to serve the webmail, other web
+servers may require more work.
+
+.. code:: nix
+
+   { config, pkgs, lib, ... }:
+
+   with lib;
+
+   {
+     services.roundcube = {
+        enable = true;
+        # this is the url of the vhost, not necessarily the same as the fqdn of
+        # the mailserver
+        hostName = "webmail.example.com";
+        extraConfig = ''
+          # starttls needed for authentication, so the fqdn required to match
+          # the certificate
+          $config['smtp_server'] = "tls://${config.mailserver.fqdn}";
+          $config['smtp_user'] = "%u";
+          $config['smtp_pass'] = "%p";
+        '';
+     };
+
+     services.nginx.enable = true;
+
+     networking.firewall.allowedTCPPorts = [ 80 443 ];
+   }

docs/index.rst

@@ -25,6 +25,7 @@ Welcome to NixOS Mailserver's documentation!
 
    backup-guide
    add-radicale
+   add-roundcube
    rspamd-tuning
    fts
    flakes