[UI]Add Mail Configuration to Configuration Page

[idanlodzki]

Description:
Add the mail (SMTP) configuration settings section to the main configuration guide —
[/docs/getting-started/configuration](https://opsimate.vercel.app/docs/getting-started/configuration).

Goal:
Include mail setup details as part of the standard OpsiMate configuration to keep all environment variables in one place and make setup clearer for users.

Notes:
Follow the same structure and tone as the other configuration sections.