docs: Fix broken links on CONTRIBUTING.md

[gonmmarques]

What does this change

Hello there, I notice this issue #2907. And indeed if you go to Porter contribute page https://getporter.org/contribute/ you will see those links leading to a 404 on GitHub.
This addresses that.

What issue does it fix

Closes #2907.

Notes for the reviewer

Let me know if I missed something as this is my first PR.

Checklist

• Did you write tests?
• Did you write documentation?
• Did you change porter.yaml or a storage document record? Update the corresponding schema file.
• If this is your first pull request, please add your name to the bottom of our Contributors list. Thank you for making Porter better! 🙇‍♀️

Reviewer Checklist

• Comment with /azp run test-porter-release if a magefile or build script was modified
• Comment with /azp run porter-integration if it's a non-trivial PR

[schristoff]

Hey @gonmmarques - thank you so much for this contributor to Porter! We've been wrestling with these 404s since we have re-organized our documentation, so I can't understate how helpful this is!