Nixpkgs porting guide

Nixpkgs is well-known for its ability to cross-compile on many platforms and compile natively on many of them. However, despite the continuous efforts and dedicated maintainer teams working to port Nixpkgs and Nix onto new platforms, there is still a lack of documentation about the process of porting.

It could be a good way to collect the domain knowledge during successful porting and encourage new contributions.

I opened a GitHub issue to keep track of this need: