Add document using file as the config center

## Why you need it?
Add document using file as the config center.
If you want to fix it, please comment "Please assign it to me" is this issue.

增加file作为配置中心的文档
如果你有兴趣修复这个问题的话, 请在下方留言 Please assign it to me

## How it could be?
A clear and concise description of what you want to happen. You can explain more about input of the feature, and output of it.

## Other related information
Add any other context or screenshots about the feature request here.

Start by reviewing the existing documentation structure in the incubator-seata-website repository and locate where configuration-center guides are kept. Clarify which file-based configuration center needs documenting, then define the required usage and expected outcome before adding the document.

Written by the indexing model from the issue text.