Add repository architecture map for new contributors

[SahilKumar75]

Summary

Add a short architecture map that helps contributors understand how the app, backend, and ML worker fit together before choosing an issue.

Scope

• Add a concise repository map to README or docs.
• Include the purpose of app, backend, ml-worker, and docs directories.
• Link the existing HLD, LLD, and contribution guide.
• Keep it short enough for first-time contributors to scan.

Acceptance Criteria

• New contributors can identify where to make changes for app, backend, ML worker, or docs work.
• Existing architecture docs are linked rather than duplicated.
• The PR includes documentation-only changes.