Docs: Update `CONTRIBUTING.md` guide with PR review expectations

[courtneypacheco]

Problem Overview

Our current CONTRIBUTING.md guide describes how to integration test a GitHub action before merging, but the Code Review section could elaborate more on testing expectations.

Expected Outcome

Update testing expectations. Think about:

• How do we want people to perform their integration tests? (We mention it in the PR template, but not elsewhere)
• Code coverage requirements when applicable
• Any language requirements?