Yeah, additional MD pages are not supported out of the box.
We had a
POC a while ago, but there were many open questions, specifically about how to combine the API reference and those MD docs and how to customize names and paths there.
At that time, it was decided not to invest more resources there because the cost vs. benefits of this approach were not clear when solutions like Writer Side, Mkdocs, and others already supported much richer functionality.
Feel free to leave your use case on the issue mentioned above