refactor: move nixos-manual to reference/ + graphifyignore

reference/nixos-manual/installation/installing-kexec.section.md

@@ -0,0 +1,61 @@
+# "Booting" into NixOS via kexec {#sec-booting-via-kexec}
+
+In some cases, your system might already be booted into/preinstalled with
+another Linux distribution, and booting NixOS by attaching an installation
+image is quite a manual process.
+
+This is particularly useful for (cloud) providers where you can't boot a custom
+image, but get some Debian or Ubuntu installation.
+
+In these cases, it might be easier to use `kexec` to "jump into NixOS" from the
+running system, which only assumes `bash` and `kexec` to be installed on the
+machine.
+
+Note that kexec may not work correctly on some hardware, as devices are not
+fully re-initialized in the process. In practice, this however is rarely the
+case.
+
+To build the necessary files from your current version of nixpkgs,
+you can run:
+
+```ShellSession
+nix-build -A kexec.x86_64-linux '<nixpkgs/nixos/release.nix>'
+```
+
+This will create a `result` directory containing the following:
+ - `bzImage` (the Linux kernel)
+ - `initrd` (the initrd file)
+ - `kexec-boot` (a shellscript invoking `kexec`)
+
+These three files are meant to be copied over to the other already running
+Linux Distribution.
+
+Note its symlinks pointing elsewhere, so `cd` in, and use
+`scp * root@$destination` to copy it over, rather than rsync.
+
+Once you finished copying, execute `kexec-boot` *on the destination*, and after
+some seconds, the machine should be booting into an (ephemeral) NixOS
+installation medium.
+
+In case you want to describe your own system closure to kexec into, instead of
+the default installer image, you can build your own `configuration.nix`:
+
+```nix
+{ modulesPath, ... }:
+{
+  imports = [ (modulesPath + "/installer/netboot/netboot-minimal.nix") ];
+
+  services.openssh.enable = true;
+  users.users.root.openssh.authorizedKeys.keys = [ "my-ssh-pubkey" ];
+}
+```
+
+
+```ShellSession
+nix-build '<nixpkgs/nixos>' \
+  --arg configuration ./configuration.nix
+  --attr config.system.build.kexecTree
+```
+
+Make sure your `configuration.nix` does still import `netboot-minimal.nix` (or
+`netboot-base.nix`).