Explicitly define scope of binderhub-service helm chart

[yuvipanda]

This helm chart has been so far primarily used to provide just access to the binderhub API. However, I primarily see it as a way to deploy binderhub in any configuration without tying it to a specific jupyterhub deployment that is contained within it (as the upstream binderhub helm chart currently is). We should explicitly document this, paying particular attention to the desire to not get entangled in jupyterhub config.