Motivation for this change

The nix build and makefile experience for the nixpkgs docs was not very nice, and extending it further was getting a bit silly.

This change replaces a bunch of annoying environment variables in the Makefile with a single nix-build and symlink. This change makes it much easier to experiment with the xsltproc parameters, as they're no longer in the environment.

This change also makes some more advanced experimentation possible via docbook customisation layers http://www.sagehill.net/docbookxsl/CustomMethods.html#CustomizationLayer

Note: each individual commit is self contained and easy to read.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.