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/10-ssl.conf.j2

@@ -2,5 +2,5 @@
 # Ansible managed: {{ ansible_managed }}
 
 ssl = {{ dovecot_ssl }}
-ssl_cert = {{ dovecot_ssl_cert }}
-ssl_key = {{ dovecot_ssl_key }}
+ssl_cert = <{{ mail_ssl_cert }}
+ssl_key = <{{ mail_ssl_key }}