I would be surprised (and disappointed) if the platform here isn't made available to all users in the coming months; while I can't be sure, the last three seem to be pure .md (with anchor links autogenerated for each section header). This would be a nice complement to the baked-in documentation functionality, as a lot of projects currently opt for the 'throw-everything-in-README.md' approach.
Shameless plug: I created a little Sublime Text 3 plugin[0] that allows to do a {stage-all/commit/pull} with a single click (the user is asked to enter a short description of the changes for the commit message); it only works _after_ basic git configuration like remote destination and password-less pushes, but there are tools that already do that for the user (e.g. github for windows). Is in beta but is already very useful with people who have issues ussing the CLI and understanding git.
We've been waiting for something like this at Thinkful for a while now. Our students struggled with learning how to host simple pages on Github so we built this interactive tutorial: http://www.thinkful.com/learn/a-guide-to-using-github-pages/
I liked the style, and also the way the selected "chapter" in the sidebar updated without breaking the back button unless you specifically jumped ahead rather than scrolled.
One suggestion would be to better differentiate between written text and the text within screen captures. Maybe a thicker border in a pale colour or some sort of banding. As it is, it's a bit harder to skim when you take the extra time tuning out diagrams, etc.
Github could potentially be disruptive in the online programming learning/tutorials space. Most projects revolve around GH in some way nowadays, it would be an interesting move for them.
