docs: Add CONTRIBUTING quick links to backend/frontend READMEs

[zhuamber370]

What

Add a small "Contributing" section to:

• backend/README.md
• frontend/README.md

The section should link to:

• CONTRIBUTING.md
• docs/contributing/dev-setup.md

Where to start

• Backend README: backend/README.md
• Frontend README: frontend/README.md

Why

New contributors often land on sub-project READMEs first. This reduces time-to-first-contribution.

Acceptance criteria

• Both READMEs contain the links
• Wording is concise and factual
• No behavior changes

Verification

• No runtime verification needed (docs-only).