[Docs]: Add docs about Style and Assets

[Timeless0911]

https://github.com/web-infra-dev/rslib/blob/main/website/docs/en/guide/advanced/css.mdx

[Timeless0911]

Should set output to web if you want to emit CSS.

[Timeless0911]

Add documentation of public dir (ref #584)