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.rst

@@ -0,0 +1,19 @@
+Roundcube
+=========
+
+Roundcube is a browser-based open-source webmail client that provides a
+full-featured email interface with support for IMAP, SMTP, address books, and
+extensible plugins.
+
+Code
+^^^^
+
+The NixOS module for Roundcube integrates almost immediately with NixOS
+mailserver, automatically configuring an Nginx virtual host and ACME-managed
+TLS for secure webmail access; using other web servers may require additional
+manual setup.
+
+Once set up you can login with your login account credentials.
+
+.. literalinclude:: ./roundcube.nix
+   :language: nix