Came across Markdoc today, made by Stripe.
Stripe is know for its excellent developer documentation - well it has to be, its probably the most important feature for the company. If you go through their documentation is not just a good reference guide, but they also allow you to interact with it to get a feel on how to use the API effectively.
Stripe built their own content authoring system and have now released this as an open source project. It supports Markdown which makes it a joy to use this tool. The tool looks great too.
The tool is so easy and pleasing to use, that I have been using this to simply write my notes 😊