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.
@BernhardKonrad @sjackman here too