This commit creates a single, central page (/doc/usb/) for all
USB-related information by combining stick-mounting.md with the
manual USB qube (formerly known as "USBVM") creation procedure
from security-guidelines.md. It rewrites the content from both of
those and adds the content requested in QubesOS/qubes-issues#1690.
It also redirects /doc/usbvm/ away from the ancient page on PVUSB.
