docs: update contribution guidelines [skip ci]

[bglamadrid]

PR Checklist

• [x] Read the contribution guidelines
• [x] Added a brief description of these changes to the top of the changelog
• [x] Changes in code meet test criteria (running mvn test returns exit code 0, without errors)

PR Type

• [x] Documentation

Summary

Improve contribution guidelines to match the existing state and recommended ways to do things here on this project. These were too outdated!