Improve CONTRIBUTING.md for Web-dev-mini-projects

The current CONTRIBUTING.md explains how to contribute, but it could benefit from examples and a step-by-step checklist for beginners.

## Suggestions
1. **Add a sample project folder structure**
- Include `index.html`, `style.css`, `script.js`, and `README.md`.

2. **Clarify branch naming and PR process**
- Give an example branch name and a simple PR title template.

3. **Add a checklist for first-time contributors**
- Fork → Clone → Add Project → Test → Commit → Push → PR

4. **Optional: Visuals**
- Screenshots or GIFs can guide beginners on how to structure files.

Improving the CONTRIBUTING.md will make it easier for new contributors to follow and increase engagement.

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