Renamed two files: right order of supported styling (#66443)
### What?
I've renamed two files:
`04-sass.mdx` -> `03-sass.mdx`
`03-css-in-js.mdx` -> `04-css-in-js.mdx`
### Why?
In the documentation of App Router/Building Your Application/Styling,
the last two items on the list (Sass, CSS-in-JS) are ordered differently
compared to the links provided below.
[Documentation](https://nextjs.org/docs/app/building-your-application/styling)
Here is the screenshot:
Co-authored-by: Sam Ko <sam@vercel.com>
