docs: copy the page about network-online.target from the wiki

docs: copy the page about network-online.target from the wiki

From https://www.freedesktop.org/wiki/Software/systemd/NetworkTarget/.

I changed the order in the page, but didn't change the text too much. Now the
discussion of the different targets is at the top, and they are ordered like
during boot (network-pre.target first, then network.target, and
network-online.target last). The parts about LSB and $network are pushed down a
bit. I think it is still useful to have them, but not as the main entry point
into the discussion. I tried to clean up the grammar and wording a bit.

One meanigful change is that we now don't say that network-online.target means
interfaces are up and IP addresses have been assigned. In other places we were
saying that the actual implementation is provided by
NetworkManager-wait-online.service, so the actual meaning is not under our
control. The text is changed to say "usually".

The last paragraph is new, I think it's good to say that
"dnf-makecache.service" is fine to use "network-online.target".