Document USB tethering as an alternative if Wi-Fi isn't working (closes: #16987)

I thought about adding more detailed instructions but I think every phone implements tethering differently. I've only tested on my phone but not on, e.g., an iPhone. When I tether to my phone, I'm given various options in addition to USB tethering. And when I do choose USB tethering, Tails gives me a wired connection. I don't know if this behavior is universal and can be documented.

Closes: #16987 (closed)