Rename Mailbox to folder in Mail documentation

[hamza221]

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• I have checked for existing issues and pull requests related to my suggestion.
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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 : nextcloud/mail#11381 / #13896 (comment)
Migrate to the UI terminology ( folder instead of mailbox) as this part for documentation is aimed for users

Additional Notes

No response