docs: add note to language and filename section (#74239)
## Description
Some people are baffled by [JavaScript files with JSX
code](https://nextjs.org/docs/community/contribution-guide#language-and-filename)
in [contribution
guide](https://nextjs.org/docs/community/contribution-guide) due to
mismatch the extension.
Therefore, add the note like the below attention.
> [!NOTE]
> - Make sure to use **`js`** extension with **JSX** code at JavaScript
files.
> - For example, ```jsx filename="app/layout.js"
### Improving Documentation
- [x] Run `pnpm prettier-fix` to fix formatting issues before opening
the PR.
- [x] Read the Docs Contribution Guide to ensure your contribution
follows the docs guidelines:
https://nextjs.org/docs/community/contribution-guide
Closes #74175
Co-authored-by: Sam Ko <sam@vercel.com>
