Rename Mailbox to folder in Mail documentation

[hamza221]

Before You Submit

• [x] I have checked for existing issues and pull requests related to my suggestion.
• [x] I have posted on the Nextcloud Community Help Forum if my suggestion is a troubleshooting scenario or I'm unsure about the solution.

Type of Suggestion

Suggesting an improvement

Relevant Documentation Links

https://docs.nextcloud.com/server/30/user_manual/en/groupware/mail.html

Summary

Rename Mailbox to folder in Mail documentation

Details

refs : https://github.com/nextcloud/mail/pull/11381 / https://github.com/nextcloud/documentation/pull/13896#discussion_r2572101204 Migrate to the UI terminology ( folder instead of mailbox) as this part for documentation is aimed for users

Additional Notes

No response