docs: add basic contributing guide

[Mahi-korrapati]

This pull request adds a basic Contributing Guide at docs/CONTRIBUTING.md.

✓ What’s included

Steps for forking and cloning the repository

Instructions for creating a new branch

Guidelines for making documentation updates

Optional testing instructions

Commands for committing and pushing changes

Instructions for opening a Pull Request

✓ Why this change is useful

The repository did not previously include a CONTRIBUTING guide, which may confuse new contributors.
This documentation helps beginners understand how to contribute and improves overall project onboarding.

[github-actions]

CLA Assistant Lite bot Thank you for your contribution! Like many free software projects, you must sign our Contributor License Agreement before we can accept your contribution.

EDIT: All contributors have signed quick-lint-js' Contributor License Agreement (CLA-v1.md).

[Mahi-korrapati]

I have read and hereby agree to quick-lint-js' Contributor License Agreement (CLA-v1.md).

[Mahi-korrapati]

Hi maintainers! The CLA is signed, and the contributing guide is added.
This PR is ready for review.
Please approve the workflows when you get a chance. Thank you!

[Mahi-korrapati]

Hi maintainers! 👋
This PR is now ready for your review.
All checks have passed, and the CLA is signed.

Since this PR adds documentation only, the workflows are currently awaiting maintainer approval.
Whenever you have time, please approve the workflows so the PR can move forward.
Thank you for your time and support!