[math4] Git tutorial

[David Farning]

Currently, there are tidbits of information on git spread around the wiki.

One of the hurdle for new contributors (of code) is figuring out how
to create an ssh key, set up an account on git.sl.o, and use git.

I would like to propose going through the wiki and moving the git
related information to a collection of three pages  Activity_Team/Git,
Activity_Team/Git_FAQ, and, Activity_Team/Git_tutorial.

david