docs: update README to reflect internal mail server scope

Refactor the documentation to accurately describe the role as a complete internal mail server stack including Postfix and Dovecot.

Changes include:
- Expanded the description to explicitly list components and internal use cases.
- Added a section clarifying excluded features (antispam/antivirus).
- Reorganized role variables into General and Postfix configuration tables.
- Cleaned up Markdown formatting and removed excessive bold styling.

templates/main.cf.j2

@@ -16,9 +16,8 @@ inet_interfaces = {{ postfix_inet_interfaces }}
 recipient_delimiter = +
 
 # TLS parameters for incoming connections
-# For a production server, replace snakeoil with real certificates.
-smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
-smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
+smtpd_tls_cert_file={{ mail_ssl_cert }}
+smtpd_tls_key_file={{ mail_ssl_key }}
 smtpd_tls_security_level=may
 smtpd_use_tls=yes