[509] Add folder structure documentation

[NanaKhadija1980j]

Summary

• Added documentation explaining the repository directory structure.
• Documented responsibilities of major directories and modules.
• Linked the guide from the main README.

Closes #509

Test plan

• Verified directory structure against the repository.
• Verified internal documentation links.
• Confirmed Markdown formatting.

Validation

• Verified every documented major directory exists on main.
• Verified relative Markdown links from docs/PROJECT-STRUCTURE.md and README resolve to existing files.
• No application logic changes.
• No markdownlint tooling is configured in this repository; validation was manual path/link checks.

Made with Cursor

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