docs: Move flashing to a separate file

doc/flashing.md

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+# Flashing firmware
+
+## Internal programmer
+
+Use this method for flashing a system already running System76 EC.
+
+```
+make BOARD=<vendor>/<model> flash_internal
+```
+
+## External programmer
+
+Use this method for first-time flashing or flashing a bricked controller.
+
+[A Mega 2560 is required for flashing.](./mega2560.md)
+
+**The system must not have any power!**
+
+1. Turn off the computer
+2. Unplug the AC adapter
+3. Remove the bottom panel
+4. Disconnect the battery
+5. Ground the laptop to the programmer
+6. Disconnect the keyboard from its port
+7. Attach the programmer to the keyboard port
+8. Flash the firmware
+
+```
+make BOARD=<vendor>/<model> flash_external
+```