docs: Move flashing to a separate file

docs/flashing.md

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+# Flashing firmware
+
+**Flashing firmware manually is *not* recommended for the normal user!**
+
+## Internal programmer
+
+Use this method for flashing a system already running System76 Open Firmware.
+
+```
+./scripts/flash.sh <model>
+```
+
+## External programmer
+
+Use one of these methods for first-time flashing or flashing a bricked system.
+
+### CH341A USB programmer - slower, but easier to set up
+
+These can be purchased from many places for around 15 USD. Make sure that the
+one you get has a ROM clip. Here is an example:
+https://www.amazon.com/Organizer-Socket-Adpter-Programmer-CH341A/dp/B07R5LPTYM
+
+**Then you can follow these steps to flash the ROM chip:**
+
+1. Turn off the computer
+2. Unplug the AC adapter
+3. Remove the bottom panel
+4. Attach the clip to the SPI ROM chip
+5. Plug the CH341A into a computer with this repository built
+6. Verify that you can access the ROM chip, replacing `<model>` with your model:
+```
+./scripts/ch341a-dump.sh <model>
+```
+7. Flash the firmware you built, replacing `<model>` with your model:
+```
+./scripts/ch341a-flash.sh <model>
+```
+
+### SPI Pi - faster, but harder to set up
+
+SPI Pi is a Raspberry Pi used to flash the SPI ROM. To create a SPI Pi:
+
+- Get a Raspberry Pi with Ubuntu LTS
+- Hook up a SPI clip to the SPI Pi
+  - The SPI Pi's SPI pinout can be found here: https://pinout.xyz/pinout/spi#
+  - The pinout of a SPI rom can be found here: https://www.totalphase.com/media/wysiwyg/soic8-pinout.png
+  - If you need more instructions, you should not be hooking up the SPI clip
+- Connect the SPI Pi's SPI clip to the SPI ROM, ensuring pin 1 of the clip aligns with pin 1 on the ROM
+- On the SPI Pi, run the following commands:
+
+```
+sudo apt install flashrom
+git clone https://github.com/system76/firmware-open.git firmware
+```
+
+**Then you can follow these steps to flash the ROM chip:**
+
+1. Turn off the computer
+2. Unplug the AC adapter
+3. Remove the bottom panel
+4. Attach the clip to the SPI ROM chip
+5. Connect to the Raspberry Pi and wait it for boot
+```
+sudo tio -b 115200 /dev/ttyUSB0
+```
+5. From the RPi session, test that the chip is detected.
+```
+cd firmware
+./scripts/spipi.sh -r backup.rom
+```
+6. From the host, flash the firmware
+```
+SPIPI=<user@spipi> ./scripts/spipi-flash.sh <model>
+```
+