Streamline and document adding new inputs to the lockfile

[thufschmitt]

The nickel lockfile (nickel.lock.ncl) only points to organist by default.
It is possible to extend it with something like that in the flake.nix file:

  outputs = {organist, myNickelLib, ...} @ inputs:
    organist.flake.outputsFromNickel ./. inputs {
      lockFileContents = {
        organist = "${organist}/lib/organist.ncl";
        myOtherInput = "${myNickelLib}/main.ncl";
      };
    };
}

However, this is

1. Not very nice (and requires re-setting the organist input as things will break otherwise)
2. Not documented