Update FirstRun Docs once dotenv is on use everywhere

[rklasen]

Once #31 and #38 are closed (happens in GH-48) the docs must be updated since most env vars must be set only once in the env var file, not system-wide anymore.

[rklasen]

Even better, dotenv itself has become somewhat less important. Now everything is in the experiment.config.

But those still have to be generated automatically, for which the paths are required.