Add boot.py docs

docs/boot.md

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+## There is a more detailed explanation in the [circuit python docs](https://docs.circuitpython.org/en/latest/README.html), however there are some common use cases for your keyboard listed here
+
+### You can hide your device from showing up as a usb storage
+
+```python
+storage.disable_usb_drive()
+```
+
+### Make your keyboard work in bios
+
+```python
+usb_hid.enable(boot_device=1)
+```
+
+### Disable serial comms
+
+```python
+# Equivalent to usb_cdc.enable(console=False, data=False)
+usb_cdc.disable()
+```
+
+### A fully working example, with a
+
+```python
+import supervisor
+import board
+import digitalio
+import storage
+import usb_cdc
+import usb_hid
+
+supervisor.set_next_stack_limit(4096 + 4096)
+
+# If this key is held during boot, don't run the code which hides the storage and disables serial
+col = digitalio.DigitalInOut(board.GP2)
+row = digitalio.DigitalInOut(board.GP13)
+col.switch_to_output(value=True)
+row.switch_to_input(pull=digitalio.Pull.DOWN)
+
+if not row.value:
+    storage.disable_usb_drive()
+    # Equivalent to usb_cdc.enable(console=False, data=False)
+    usb_cdc.disable()
+    usb_hid.enable(boot_device=1)
+
+row.deinit()
+col.deinit()
+```