Backport #25417 by @wxiaoguang
Update WorkPath/WORK_PATH related documents, remove out-dated
information.
Remove "StaticRootPath" on the admin config display page, because few
end user really need it, it only causes misconfiguration.
![image](https://github.com/go-gitea/gitea/assets/2114189/8095afa4-da76-436b-9e89-2a92c229c01d)
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>