doc: Explain how to hack on kernel

Presents the options available (linuxManualConfig versus overriding
extraConfig, ignoreConfigErrors, autoModules, kernelPreferBuiltin.

For advanced hostPlatform customization refer to the commands shared by ericson1234 at
https://github.com/NixOS/nixpkgs/pull/33813 but it is too advanced to
put in the doc.