Add zmk_usb_boot toggle to documentation

[typorian]

The |CONFIG_ZMK_USB_BOOT switch was missing from the documentation.

Board/Shield Check-list

• This board/shield is tested working on real hardware
• Definitions follow the general style of other shields/boards upstream (Reference)
• .zmk.yml metadata file added
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• General consistent formatting of DeviceTree files
• Keymaps do not use deprecated key defines (Check using the upgrader tool)
• &pro_micro used in favor of &pro_micro_d/a if applicable
• If split, no name added for the right/peripheral half
• Kconfig.defconfig file correctly wraps all configuration in conditional on the shield symbol
• .conf file has optional extra features commented out
• Keyboard/PCB is part of a shipped group buy or is generally available in stock to purchase (OSH/personal projects without general availability should create a zmk-config repo instead)

[caksoylar]

Thanks for noticing and submitting a fix!