Improve README and CONTRIBUTING to lower onboarding barrier for new contributorsdocs: improve onboarding with quick start and contributor guidance

[pranjal2004838]

Summary

This PR improves newcomer onboarding by adding clear, low-barrier entry points
to both README.md and CONTRIBUTING.md, while preserving existing detailed
documentation for advanced users.

Changes Made

README.md

• Added a Quick Start for Beginners section at the top (after the Gitpod button)
  with three easy setup options:
  • Gitpod (one-click browser-based setup)
  • Docker (single-command local setup)
  • Manual setup (links to detailed instructions)
• Updated the Table of Contents to include the new Quick Start section
• Added a note in the Installation section directing beginners to Quick Start first

CONTRIBUTING.md

• Reorganized content to lead with a welcoming introduction and Code of Conduct
• Added a Quick Start for Contributors section outlining:
  • Project setup
  • Finding issues
  • Making contributions
  • Getting help
• Preserved and clarified existing links to first-timer issues and contributor resources

Motivation

As a new contributor exploring this repository, I noticed that while the existing
documentation is thorough, it can feel overwhelming at first. These changes aim
to make the first 5–10 minutes smoother for newcomers (especially via Gitpod),
while keeping full documentation available for experienced contributors.

This aligns with Public Lab’s goal of being accessible and welcoming to new
contributors.

Notes

• No functionality changes
• Documentation-only improvements

Happy to adjust wording or structure based on maintainer feedback.

[pranjal2004838]

It looks like some CI checks are failing, but this PR only includes documentation
changes (README.md and CONTRIBUTING.md).

Please let me know if you’d like me to rebase or make any adjustments on my side.
Happy to update as needed.