nixpkgs deprecated neovimUtils.makeNeovimConfig in favor of
wrapNeovimUnstable. Keep Nixvim's current generated init.lua startup
model, but stop routing wrapper construction through the deprecated
helper.
Switch modules/top-level/output.nix to call wrapNeovimUnstable
directly and preserve the existing wrappedNeovim.initRc -> generated
init.lua composition. This aligns Nixvim with the upstream wrapper API
change without changing our current bootstrap behavior yet.
Also update CONTRIBUTING to point contributors at
wrapNeovimUnstable instead of the deprecated helper.