docs: update Roundcube guide

Adds a short explanation what roundcube even is.

Extract and extend the roundcube example showing plugin and spellchecking
support. We also inherit a plausible maximum attachment size based on
Postfix's message_size_limit. The nginx vhost forces TLS and manages
certificates using the ACME integration.

docs/roundcube.nix

@@ -0,0 +1,41 @@
+{
+  config,
+  pkgs,
+  ...
+}:
+
+{
+  services.roundcube = {
+    enable = true;
+    hostName = "webmail.example.com"; # the nginx vhost
+    package = pkgs.roundcube.withPlugins (
+      plugins: with plugins; [
+        # https://search.nixos.org/packages?query=roundcubePlugins
+        persistent_login
+      ]
+    );
+    dicts = with pkgs.aspellDicts; [
+      # https://search.nixos.org/packages?query=aspellDicts
+      en
+    ];
+    # Account for ~30% size increase due to base64 encoding of attachments
+    # https://github.com/roundcube/roundcubemail/issues/7979
+    maxAttachmentSize = config.mailserver.messageSizeLimit / 1024 / 1024 / 1.37;
+    extraConfig = ''
+      $config['imap_host'] = "ssl://${config.mailserver.fqdn}";
+      $config['smtp_host'] = "ssl://${config.mailserver.fqdn}";
+      $config['smtp_user'] = "%u";
+      $config['smtp_pass'] = "%p";
+    '';
+  };
+
+  services.nginx.virtualHosts.${config.services.rounducbe.hostName} = {
+    enableACME = true;
+    forceSSL = true;
+  };
+
+  networking.firewall.allowedTCPPorts = [
+    80
+    443
+  ];
+}