Nix infrastructure is poorly documented

[wucke13]

I'd like to use the existing Nix build-scripts, but they are quite far from common conventions w/r/t structuring. It would be good to either document the structure chosen more elaborately, or, flatten it out. Flakes could be a good (optional?!) entrypoint so that all buildable derivations (with attr path) and description of the various items can be easily discovered. Also that might ease CI setup.