Add Rstudio documentation

[konradpa]

This adds documentation for the Rstudio GUI for the basic git functions (First steps and Branches chapter) #223

[lnnrtwttkhn]

• I think screenshots would make this even better. I think it would be good, if we come up with a "standardized" way to add screenshots in this case, e.g., same look and layout of the RStudio IDE etc., ideally in a reproducible way.

[konradpa]

[x] I think screenshots would make this even better. I think it would be good, if we come up with a "standardized" way to add screenshots in this case, e.g., same look and layout of the RStudio IDE etc., ideally in a reproducible way.

I updated the screenshots to ones taken inside the docker container