[Documentation] Update code contribution guidelines

[fraser-combe]

🗑️ This dev branch should be deleted after merging to main.

🧠 Summary

This is just an update to the code contribution documentation that may help new users. Feel free to amend and update

🔬 Final Developer Checklist

• The workflow/task has been tested and results, including file contents, are as anticipated
• The CI/CD has been adjusted and tests are passing (Theiagen developers)
• Code changes follow the style guide
• Documentation and/or workflow diagrams have been updated if applicable (Theiagen developers only)

🎯 Reviewer Checklist

• All changed results have been confirmed
• You have tested the PR appropriately (see the testing guide for more information)
• All code adheres to the style guide
• MD5 sums have been updated
• The PR author has addressed all comments
• The documentation has been updated

[sage-wright]

Great additions! ⭐