What tool does Zendesk use to author its API reference documentation?

Hi Kristin! It's actually an in-house solution that one of our teams here created. In a nutshell, we have an internal tool that handles our OpenAPI Spec, which then produces Markdown files, that are then generated in a neat and clean format using Gatsby. I wish this was an open source option that we could share, but there are lots of great API docs tools out there so hopefully you find one that works for you!