Add documentation for config.json

[vogtmh]

Is your feature request related to a problem? Please describe.
So I downloaded the latest release to my webserver and used the provided config.sample.json as config.json - all good. However, I would like to customize my installation now and don't know how do it.

Describe the solution you'd like
A simple documentation about the supported options in the config.json

Describe alternatives you've considered
I was already looking into altering the JS files myself, but it's a bit overwhelming when you just started out :D

Additional context
My goal is to disable all features that I do not necessarily require, such as SSO auth and many style options. In the end, the client might be able to run on some of my older devices, too.