[Docs] How to use git at coffea-casa from JupyterHub

[oshadura]

@mat-adamec we need to add a section about using git from AF for both cases: from terminal and from Jupyterhub Git/GH plugins https://github.com/jupyterlab/jupyterlab-git

[mat-adamec]

Do we feel like the current section about git for Jupyterhub is satisfactory?

If so, I think only the terminal section above is relevant. ~~We can handle that when https://github.com/CoffeaTeam/coffea-casa/issues/49 is figured out.~~

Actually, on second thought, Git is still completely usable from terminal even if we can't run full analyses in it. Okay, I can write documentation for that, but it feels so basic so as to be outside the scope of coffea-casa. Do we really need a full tutorial on how to use git from the terminal? Plenty exist online! :)