docs: add note to language and filename section

[JamBalaya56562]

Description

Some people are baffled by JavaScript files with JSX code in 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

• Run pnpm prettier-fix to fix formatting issues before opening the PR.
• Read the Docs Contribution Guide to ensure your contribution follows the docs guidelines: https://nextjs.org/docs/community/contribution-guide

Closes #74175

[ijjk]

Allow CI Workflow Run

• approve CI run for commit: 12ca6c5

Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer