Update Contributing Docs

[zacdezgeo]

What I changed

• Removed the outdated reference to the preview.yml GitHub Actions workflow from contributing.qmd.
• Updated the instructions to reflect that PR previews are now handled via Netlify, with a bot posting the preview link automatically.

How to test it

• Run quarto preview locally or check the Netlify deploy preview.
• Navigate to the "Get Involved" page.
• Confirm that:
  • There is no mention of preview.yml or GitHub Actions.
  • The instructions clearly explain that previews are generated by Netlify and a bot comments with the link.

Other notes

• This change aligns the contribution documentation with the deployment updates in Publish previews using netlify Publish previews using netlify #166
• Closes Clean Up Contribution docs Clean Up Contribution docs #168