Yes, you can; the ROM contains a driver that can flash the device over USB. You do need a way to be able to put it into download mode by grounding GPIO0, though.
I'm not sure if there's a page describing this aside from mentions that you can also flash over USB here and there.What code example would you need? The USB port exposes an industry-standard CDC-ACM serial port (over which you can use the same protocol as when you use UART) and DFU endpoints.