refactor: move nixos-manual to reference/ + graphifyignore

reference/nixos-manual/installation/obtaining.chapter.md

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+# Obtaining NixOS {#sec-obtaining}
+
+NixOS ISO images can be downloaded from the [NixOS download
+page](https://nixos.org/download.html#nixos-iso). Follow the instructions in
+[](#sec-booting-from-usb) to create a bootable USB flash drive.
+
+If you have a very old system that can't boot from USB, you can burn the image
+to an empty CD. NixOS might not work very well on such systems.
+
+As an alternative to installing NixOS yourself, you can get a running
+NixOS system through several other means:
+
+-   Using virtual appliances in Open Virtualization Format (OVF) that
+    can be imported into VirtualBox. These are available from the [NixOS
+    download page](https://nixos.org/download.html#nixos-virtualbox).
+
+-   Using AMIs for Amazon's EC2. To find one for your region, please refer
+    to the [download page](https://nixos.org/download.html#nixos-amazon).
+
+-   Using NixOps, the NixOS-based cloud deployment tool, which allows
+    you to provision VirtualBox and EC2 NixOS instances from declarative
+    specifications. Check out the [NixOps
+    homepage](https://nixos.org/nixops) for details.