Fix documentation of configuring-dcache

[SarahAlidoost]

The documentation of configuring-dcache mentiones:

on the platform:

mkdir -p ~/.config/fsspec
cp ./config/fsspec/config.json ~/.config/fsspec/

However, the updated config.json file is located on the local system and scp command should be used to copy files. Also, the command mkdir only works on the platform.

[fnattino]

As commented in #77, we could then maybe moving towards specifying all these parameters via environment variables? This way we would not need to bother about this sort of issues..