linuxManualConfig involves more boilerplate to change the kernel. Use
the wrapper linuxPackages_custom which is wrapper that takes an
attribute sets and calls linuxManualConfig approrpriately.
This is much easier for beginners to use instead of linuxManualConfig
helper.
Point to linuxManualConfig for further customizations.
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.
