Add full list of available Environment Variables

[swedishstyle]

Is your feature request related to a problem? Please describe.
There is no complete list of usable environment variables, I had to guess by trial and error from a vague description with a caveat on one page of the documentation

Describe the solution you'd like
A page with a full and complete list of acceptable environment variables and what they do

Describe alternatives you've considered
Nil

Additional context
I'd be happy to write it up (assuming I can list them correctly), but I couldn't see any way to contribute to the docs I found at https://maxdorninger.github.io/MediaManager/configuration.html

[maxdorninger]

IIRC all env variables should be in the docs. You can contribute to the docs by editing the markdown files in the /docs folder. I explained it in a bit more detail here: https://maximilian-dorninger.gitbook.io/mediamanager/master/contributing-to-mediamanager/documentation