Remove references to developer docs
This hasn't been available for a while now. And was not updated
for even longer before that. Let's not mislead our users any
longer.
[Documentation (Releases)](https://docs.rs/nix/)
-[Documentation (Development)](https://nix-rust.github.io/nix/nix/index.html)
-
Nix seeks to provide friendly bindings to various *nix platform APIs (Linux, Darwin,
...). The goal is to not provide a 100% unified interface, but to unify
what can be while still providing platform specific APIs.
