I don't understand why peoples provide documentation throught github, which is more suitable for source code (or source code of documentation).

I don't understand why people complain about doing something one way without offering a better alternative. No value is added and no discussion can be had.

Is there a better alternative to github for this source of information? I've got this crazy idea to write a technical book with TeX. Would github be a good place to keep it along with my example code?

wiki

Might be a good place to start. Is there a free place where I can start hosting it? I hope to eventually have a real book that I can publish. Any tricks on getting the output formatted nicely?

Github has wikis.

1) It's there, free and available and works pretty well. You don't have to worry about setting up a separate site, etc.

2) Being able to fork, patch, feedback & improve documentation is a good thing :-)

Did you look at any of the files? It's not documentation, it's example code.

Nice. It is actually quite important to understand view-ports to efficiently design your web-apps these days.

I think it would be nice to add definitions about css pixels, device pixels and scaling also, to help understanding how @media-queries or head declarations would work.