refactor: move nixos-manual to reference/ + graphifyignore

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

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+# Booting from a USB flash drive {#sec-booting-from-usb}
+
+The image has to be written verbatim to the USB flash drive for it to be
+bootable on UEFI and BIOS systems. Here are the recommended tools to do that.
+
+## Creating bootable USB flash drive with a graphical tool {#sec-booting-from-usb-graphical}
+
+Etcher is a popular and user-friendly tool. It works on Linux, Windows and macOS.
+
+Download it from [balena.io](https://www.balena.io/etcher/), start the program,
+select the downloaded NixOS ISO, then select the USB flash drive and flash it.
+
+::: {.warning}
+Etcher reports errors and usage statistics by default, which can be disabled in
+the settings.
+:::
+
+An alternative is [USBImager](https://bztsrc.gitlab.io/usbimager),
+which is very simple and does not connect to the internet. Download the version
+with write-only (wo) interface for your system. Start the program,
+select the image, select the USB flash drive and click "Write".
+
+## Creating bootable USB flash drive from a Terminal on Linux {#sec-booting-from-usb-linux}
+
+1. Plug in the USB flash drive.
+2. Find the corresponding device with `lsblk`. You can distinguish them by
+   their size.
+3. Make sure all partitions on the device are properly unmounted. Replace `sdX`
+   with your device (e.g. `sdb`).
+
+  ```ShellSession
+  sudo umount /dev/sdX*
+  ```
+
+4. Then use the `dd` utility to write the image to the USB flash drive.
+
+  ```ShellSession
+  sudo dd bs=4M conv=fsync oflag=direct status=progress if=<path-to-image> of=/dev/sdX
+  ```
+
+## Creating bootable USB flash drive from a Terminal on macOS {#sec-booting-from-usb-macos}
+
+1. Plug in the USB flash drive.
+2. Find the corresponding device with `diskutil list`. You can distinguish them
+   by their size.
+3. Make sure all partitions on the device are properly unmounted. Replace `diskX`
+   with your device (e.g. `disk1`).
+
+  ```ShellSession
+  diskutil unmountDisk diskX
+  ```
+
+4. Then use the `dd` utility to write the image to the USB flash drive.
+
+  ```ShellSession
+  sudo dd if=<path-to-image> of=/dev/rdiskX bs=4m
+  ```
+
+  After `dd` completes, a GUI dialog "The disk
+  you inserted was not readable by this computer" will pop up, which can
+  be ignored.
+
+  ::: {.note}
+  Using the 'raw' `rdiskX` device instead of `diskX` with dd completes in
+  minutes instead of hours.
+  :::
+
+5. Eject the disk when it is finished.
+
+  ```ShellSession
+  diskutil eject /dev/diskX
+  ```