The Nixpkgs documentation on the linux kernel builders focused on
using and extending kernels that were already packaged, but never
mentioned that it's possible to also build a kernel almost "from
scratch".
The NixOS documentation went a bit deeper on manual linux kernel
configs, but that information wasn't particularly NixOS-specific.
This commit consolidates the information related to building the
kernel on Nixpkgs's documentation, while keeping any additional
NixOS-specific information on NixOS's documentation.
An additional README.md was created for contributor-facing
documentation.
