Flatdoc seems useful and clean. I'm in the process of cleaning up & packaging Sinatra/Sphinx/email app for a semi-technical set of users and this seems a bit clearer and easier for them to navigate. Nothing replaces actually reading the whole README, but if you're not used to it I think it can be a bit intimidating for some people. I especially like the zero server side aspect, I'm trying to keep the dependencies for my app as minimal as possible for me.
Looks cool! I've written a much simpler version of this (basically a shell script that calls make, with a separate makefile that calls pandoc) specifically for github pages: https://github.com/lvh/typewriter
This definitely comes with a lot more bells and whistles out of the box though :)
It took me a couple seconds to figure out there was content below the auto-resizing header image. I tried to click on both the "Documentation is easy" and "FLATDOC" text to see if either were links to the content before realizing there was a scroll bar.
I'd suggest not having such a large image, or at least resize it to only expand to a portion of the screen, to prevent confusion.