nixos/doc/releases: update the docs as promised
nixos/doc/releases: update the docs as promised

This goes through a recent example of 19.09 (because the workflow
should be everchanging, so our example needs to be recent).
Lots of changes, just read idk.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs