[zh_CN] Translation of FireFly documentation for Chinese, Simplified

### What would you like to be added?

- Translations of doc-site to Chinese, Simplified.
- as this is the first translation, a [language switcher](https://discord.com/channels/905194001349627914/1325036925660172288/1326451413110231082) in the documentation site will need to be added, and the folder layout will need to be proposed

### Why is this needed?

as well put by @Shawn-Shaw-x in https://discord.com/channels/905194001349627914/1154921177207275520/1325036925660172288:

`
Many developers in China mainland who are not familiar with english. They also want to learn this project or to make their own contributions on this project.
`