eta: Having now had a closer look, some more thoughts:
The tool is clearly focussed on writing docs "for a website"; for external consumption. (For whatever value you choose for "external".) Definitely not for internal-to-a-project docs that you'd want side-by-side in a code project[1]. So no wikilinking between pages, which is (for me) a deal-breaker, making this thing basically just a previewing Markdown editor that also does XML source.
Well, it's early days, so let's hope that the tool becomes all it might be someday. But if you're doing "for external consumption via a website" docs, then maybe this can be useful to you, though it's hard to see what it's doing that a dozen other previewing Markdown editors don't already do, and some better.
[1] Yes, I'm aware that there are several plugins to IJ that do side-by-side note-making in projects. I've tried (probably) all of them, and they're all badly deficient in one way or another.