docs: translate the Chinese doc of backup.md into English

[lumianph]

• The source of the Chinese docs is here: https://github.com/4paradigm/OpenMLDB/blob/main/docs/zh/maintain/backup.md
• The translated English doc should be saved under the path of docs/en/maintain/backup.md
• add the page entry to the file docs/en/maintain/index.rst

[yclchuxue]

I want to do this.

[lumianph]

I want to do this.

Thank you for the interest. I have assigned it to you. Thanks.