[$] Nix alternatives and spinoffs

Since thedisagreements that led to Eelco Dolstrastepping down from the NixOSFoundation board, there have been a number of projects forked from or inspiredbyNix that have stepped up to compete with it. Two months on, some of theseprojects are now well-established enough to look at what they have to offer andhow they compare to each other. Overall, users have a number of good options tochoose from, whether they're seeking a compatible replacement for Nix (theconfiguration language and package manager) or NixOS (the Linux distribution),or something that takes the same ideas in a different direction.