gitbook editor... https://www.gitbook.com/editor
other solutions less easy to deal with...www.doxygen.com
but worthy because of source code cross-reference and a ton of add ons for many different languages
I use both.
Gitbook for introductory material, research, etc...
doxygen for actually linking from source code back into gitbook via links
doxygen also integrates with MSC http://www.mcternan.me.uk/mscgen/
which I have found quite useful for documenting the sycall back-and-forth, etc....