Hey, I work on the Docs Product engineering team at Stripe and created Markdoc.

The scope is somewhat different: Jekyll and Middleman are static site generators that provide a full stack for content publishing whereas Markdoc is a framework for processing Markdown content with custom tags and transforming it into the desired output format. Markdoc can be used inside of a static site generator or a custom documentation platform.

Markdoc's syntax and custom tags also take a somewhat different approach than many of the string templating systems that are commonly used in static site generators. Markdoc is not a preprocessor, the tags are treated as a first-class part of the markdown content, which parses into a data structure that can be statically analyzed, programmatically transformed, and rendered into the desired output format. For us, that's React, but it could be raw HTML, Web Components, a slide deck, or whatever else you might come up with.

Upon further inspection, I think the biggest difference is that Markdoc uses Nextjs which means you can write React right in the middle of your code and import React components. But then again, you have Gatsby that's also based on React and is built for static site generation, and supports Markdown.