Document lefthook and other tests

What does this MR do and why?

• Adds lefthook as a dependency
• Copies over the testing doc page, with a few edits (e.g, we test Go now, not Ruby)

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.