Reorganize Git install, setup, test-drive and daily-usage instruction.

This is @BernhardKonrad 's proposal for removing the duplication that gradually evolved with our introductory Git(Hub) materials.

Original pull request was from his own fork. I made new branch on our main repo (`bernhard-DRY-git`), with same parent as his, and merged in his commit.

This new pull request is starting exactly where he did.

I plan to modify this branch, which will update this pull request, and eventually merge from here.

From @BernhardKonrad 's original PR:

See [rendered version](http://bernhardkonrad.github.io/STAT545-UBC.github.io/).

Check the new [hub for all git-related install, set-up, and workflow](http://bernhardkonrad.github.io/STAT545-UBC.github.io/github.html).

The goal is to close STAT545-UBC/InstructorsOnly/#15, STAT545-UBC/InstructorsOnly/#16, STAT545-UBC/InstructorsOnly/#17.
I'm not entirely sure I get your point. The [Linux help](https://help.github.com/articles/caching-your-github-password-in-git#platform-linux) says
> Turn on the credential helper so that Git will save your password in memory for some time. By default, Git will cache your password for 15 minutes.
Maybe this is not necessary on a Mac, @sjackman could confirm this. And for Windows, could you test this @jooolia?