There aren’t any official instructions from https://www.qubes-os.org/; shouldn’t there be a page designated to this or even for the wifi?
Did a search at the bottom and nothing for internet or ethernet.
Found 1 article on the internet but it is for setting up the wifi, but it seems the writer was writing based on discovery, so not sure it would be the right way to do it or just the writer’s luck, coincidence, or guess.
Where are the official instructions to setup the ethernet to the internet?
It should work in the same way as with a regular Linux distro. You setup your ethernet network in sys-net, and the latter provides it to other VMs (via sys-firewall by default). The same NetworkManager icon should provide the UI for the ehternet AFAIK.
Do you have any specific questions?
This page of QubesOS documentation describes how to use PCI devices. The ethernet PCI device of your computer should have been automatically assigned to your sys-net qube when you installed QubesOS. If this is the case, plugging in an ethernet cable should provide your sys-net qube with network connectivity almost immidiately. By default, sys-firewall receives network connectivity from sys-net and then provides network connectivity for everything else that needs it. If your ethernet PCI device is not attached to your sys-net qube, you can attach it manually using the Qubes Manager or by using the terminal command, both explained on the linked page above.