[Documentation] Add default values to environment variables

[T3rm1]

I wanted to create a minimal setup and was overwhelmed by the amount of configuration options in the .env file. Unfortunately neither the example file nor the documentation page states what the default value of each parameter is.

My goal was to start with an empty env file and only change what is necessary - to keep it simple. It's crucial to know what the default for various parameters is, e.g. MONGO_MAX_POOL_SIZE.

At first, I thought that the example might be also the default value but that is not the case. I tested it by removing PORT from the file and the application would no longer start.