Installation documentation improvement about installing in jupyterhub and/or jupyter server environment

[dirkdaems]

While reading the documentation, it is not clear to me whether this extension needs to be installed on the Jupyterhub only. I can imagine that the extension also needs to be installed in the JupyterLab Docker container that is being spawned (using k8s, Docker Swarm, ...) as the repository needs to become available in that environment?
Can this be clarified?

[consideRatio]

I've not read the docs myself recently, but if you've read it and can't say clearly where it needs to be installed - i think its worth clarifying it in the docs. I've "documentation" and "help wanted" labels for this issue.

The python environment running JupyterHub itself doesn't need nbgitpuller installed. Only the Python environment that starts a jupyter server (jupyter_server or notebook) needs to have it installed, because nbgitpuller its a jupyter server extension. It shouldn't matter if the jupyter server is started via jupyterhub or not.