Well a core point about the GoW wiki is that it will NOT teach the use of programs used in the creation of Ages, development, etc.. So this includes teaching C++, Blender, Visual Studio... and Git.
But don't worry. I learned Git on the fly as I was experimenting with Plasma. Just make a few quick searches on the Internet and you'll have everything you need. Here are the basics:
1. When installed on Windows, it should add a right-click context entry for the "Git bash", which is a terminal capable of interpreting Git commands.
2. Common commands are: "git clone" which clones a remote repository, "git commit" which takes the changes you've made locally and registers them and "git push" which pushes your commited changes remotely (to a repository on which you have write permissions).
3. Github is a hosting platform for Git repos, and a very convenient one, thanks to a lot of neat "social" features - but you can absolutely host your own Git server, at the cost of a loss of more advanced features.