[ENH] add first draft lightweight git tutorial

[slumnitz]

• add instructions on how to install git for MacOS and Windows
• add instructions on how to set up git for BioPAL collaboration
• add code example of git workflow

To-Do:

• [ ] add instructions for installation with Linux (low_priority)
• [ ] add images to illustrate Github functionality

[slumnitz]

@pmazzucchelli content of this PR were merged with #6 as #6 was build on top of #5.

Hence the git-tutorial content was merged with PR #6

I personally get asked more often about how to use git than how to contribute to docs and think that even if not a priority both are useful here.