Operator env vars documentation

[fhennig]

currently there is hand written docs for env vars and commandline parameters, but that in itself isn't useful.

The users want to set this with Helm, or somehow in OpenShift (or maybe with stackablectl?)

These things should go into the Reference section.

We should also think about maybe generating this if we can (but not necessary, as there are probably not that many settings). Maybe we can have instead common operator flags as a partial somewhere, that we can simply reference in all the operators.

related: #514