Improve CONTRIBUTING.md for first-time contributors

[RitikaSen27]

This PR improves CONTRIBUTING.md to make onboarding easier for first-time contributors. It adds a quick links section, step-by-step guide, and PR workflow instructions.

Fixes #11846

This PR improves CONTRIBUTING.md to make onboarding easier for first-time contributors.

Changes include:

• Added a Quick Links section for essential pages
• Step-by-step guide for first-time contributors
• Clear Pull Request workflow instructions

These changes address issue #11846 and make it easier for new contributors to start contributing to PublicLab projects.

The screenshots of the changes done in the CONTRIBUTING file are given below:

Looking forward to feedback!

[welcome]

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[RitikaSen27]

Screenshots for both README.md and CONTRIBUTING.md have been added.

This PR contains documentation-only changes, and CI failures appear unrelated.
Thanks for reviewing!