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.

defaults/main.yml

@@ -58,8 +58,8 @@ dovecot_mail_location: "maildir:~/Maildir"
 # SSL/TLS configuration
 # Use 'yes', 'no' or 'required'. 'required' is recommended for production.
 dovecot_ssl: "yes"
-dovecot_ssl_cert: "</etc/ssl/certs/ssl-cert-snakeoil.pem"
-dovecot_ssl_key: "</etc/ssl/private/ssl-cert-snakeoil.key"
+mail_ssl_cert: "/etc/ssl/certs/ssl-cert-snakeoil.pem"
+mail_ssl_key: "/etc/ssl/private/ssl-cert-snakeoil.key"
 
 # Authentication mechanisms
 dovecot_auth_mechanisms: "plain login"