Updated config with steps to verify and some other bits and pieces

Updated the guide to include instructions on how to verify the Ubuntu image, as well as clarify the steps for network configuration and user creation.
Is this true for all servers? I usually hit F10 for the NUCs, since that's what it tells me to do. Do any of these other keys work on the NUCs, for example?

The "every server model is different" bit sort of seems like it tries to explain the potential ambiguity, but I don't think it is clear. How about: "Start by inserting the Ubuntu installation media into the server. Boot or reboot the server with the installation media inserted, and enter the boot menu. To enter the boot menu, you need to press a key as soon as you turn the server on. This key varies depending on server model, but common choices are `Esc`, `F10`, and `F12`. Often, the server will briefly display a message on boot that shows which key should be pressed to enter the boot menu. Once you've entered the boot menu, select the installation media (USB or CD) and press Enter to boot it.`