Version Control -> Workflow should explicitly mention using a staging server

[brandwaffle]

Currently we allude to using a staging server in our version control workflow section https://10up.github.io/Engineering-Best-Practices/version-control/. That said, we don't explicitly discuss staging servers themselves, or why they're part of a solid process for deploying code and ensuring site reliability. We should directly address staging/testing configs, including why they're critical, as part of our best practices.