I was wondering why mdBook wasn't being used for documentation and found @cart commented about this on Discord. I'll quote what he said for anyone else wondering and maybe we can discuss the idea.

Quote of @cart's message on Discord:

I've been playing around with that idea for awhile. It's clearly the more feature complete solution, but I don't want to lose the "integrated feel" of the current implementation. I also think using the same codebase for all website contributions keeps things nice and simple.

It's worth opening an issue to capture this

For myself the "integrated feel" doesn't matter with the documentation. Many awesome open source projects have a separate docs website which I don't mind at all. Bevy could have a docs.bevyengine.org where the documentation would reside at or maybe book.bevyengine.org which seems to be the style for rust projects.