refactor: move nixos-manual to reference/ + graphifyignore

reference/nixos-manual/installation/installing-virtualbox-guest.section.md

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+# Installing in a VirtualBox guest {#sec-installing-virtualbox-guest}
+
+Installing NixOS into a VirtualBox guest is convenient for users who
+want to try NixOS without installing it on bare metal. If you want to set
+up a VirtualBox guest, follow these instructions:
+
+1.  Add a New Machine in VirtualBox with OS Type "Linux / Other Linux"
+
+1.  Base Memory Size: 768 MB or higher.
+
+1.  New Hard Disk of 10 GB or higher.
+
+1.  Mount the CD-ROM with the NixOS ISO (by clicking on CD/DVD-ROM)
+
+1.  Click on Settings / System / Processor and enable PAE/NX
+
+1.  Click on Settings / System / Acceleration and enable "VT-x/AMD-V"
+    acceleration
+
+1.  Click on Settings / Display / Screen and select VMSVGA as Graphics
+    Controller
+
+1.  Save the settings, start the virtual machine, and continue
+    installation like normal
+
+There are a few modifications you should make in configuration.nix.
+Enable booting:
+
+```nix
+{ boot.loader.grub.device = "/dev/sda"; }
+```
+
+Also remove the fsck that runs at startup. It will always fail to run,
+stopping your boot until you press `*`.
+
+```nix
+{ boot.initrd.checkJournalingFS = false; }
+```
+
+Shared folders can be given a name and a path in the host system in the
+VirtualBox settings (Machine / Settings / Shared Folders, then click on
+the "Add" icon). Add the following to the
+`/etc/nixos/configuration.nix` to auto-mount them. If you do not add
+`"nofail"`, the system will not boot properly.
+
+```nix
+{ config, pkgs, ... }:
+{
+  fileSystems."/virtualboxshare" = {
+    fsType = "vboxsf";
+    device = "nameofthesharedfolder";
+    options = [
+      "rw"
+      "nofail"
+    ];
+  };
+}
+```
+
+The folder will be available directly under the root directory.