Changes chapter 3 (GitHub)

[stephaneguerrier]

• [x] Remove github classroom
• [x] Add text + video on branching
• [x] Add comics and adapt text
• [x] Add text + video for push, pull
• [x] How to use github in rstudio
• [x] Add someting (text + video) on github desktop
• [x] Add more details on slack + github (video?)
• [x] Add something on issues, I think this is important so that students will be able to add issues to this project.
• [x] Add something on merge conflict, i.e. explain what to do with :

<<<<<<<
HEAD
some text
=======
some other text
>>>>>>>
some numbers