Update installation-guide.md : disconnect all USB devices prior of stage2 install

[tlaurion]

Otherwise detected USB HID devices (keyboard) will be permitted to interact with dom0 as security baseline

Closes linuxboot/heads-wiki#171
Closes QubesOS/qubes-issues#9743

[marmarek]

This wants two important clarifications:

1. If your only/primary keyboard/mouse is on USB, of course don't disconnect it.
2. The presence of USB keyboard affects only presented default configuration. Alternatively to disconnecting all of that, you can simply uncheck "allow usb keyboard" checkbox (if you don't want to use USB keyboard). This one: https://openqa.qubes-os.org/tests/126419#step/firstboot/9. Below the option you can see list of devices that will be affected.