docs: add CONTRIBUTING.md with local dev setup guide for web and mobile

[Nareshkumawat-star]

What's Missing

The DevCard repo has no CONTRIBUTING.md. New contributors have no guide on how to set up the project locally, which platform uses which tech stack, or what the PR process looks like.

Proposed Sections

1. Project overview: monorepo structure, what web, mobile, backend folders contain
2. Prerequisites: Node.js version, Expo CLI, backend runtime
3. Local setup: step-by-step clone → install → env vars → run for each platform
4. Branch naming conventions: feat/, fix/, docs/
5. PR checklist
6. Issue labels guide

File to Create

CONTRIBUTING.md in repo root

[github-actions]

Hi @Nareshkumawat-star,

Thanks for opening this issue.

Please reply with one of the following areas so the issue can be routed to the appropriate team member:

- /backend
- /web
- /mobile
- /devops

Once an area is selected, the corresponding label will be added automatically.

[Nareshkumawat-star]

/backend

[github-actions]

The issue has been classified as backend.

@Harxhit, please review and triage this issue when available.

The backend label has been applied and the issue has been routed accordingly.