docs: clarify where to add custom CSS files

[atharv96k]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

This change addresses #12031.

Beginner users often struggle to find where to place custom CSS files when customizing a Docusaurus site. Added a project folder structure example under the Global Styles section to make this clearer.

Test Plan

This is a documentation-only change. No code was modified.

Verified that the added folder structure renders correctly in the MDX file.

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/docs/styling-layout

Related issues/PRs

Closes #12031

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