A quick update from the Jupyter Book team (which is not technically part of Jupyter, though there’s an open JEP to re-incorporate as a Jupyter subproject) 
Until now, the Jupyter Book project was built on top of Sphinx. We’re excited to announce that we plan to rebuild it on top of the MyST Markdown document engine!
This will enable more modern workflows and integration with other tools in the Jupyter ecosystem. See this post for more information👇