@elena Joke aside, I’ve read about half of it and see nothing to compjain about as a SysAdmin ;)
One thing that could make it even better: I’m a huge fan of documentations that explain why before how. I think it would be a great addition if you could add 2 or 3 lines at the beginning of each steps to tell your reader why she’ll have to follow those steps.
See for example step 1, you dive into the how right from the start and it’s only after 5 sub-steps and 5 corresponding screen captures that you explicit the why «When the process completes, your new subdomain - which you will use for NextCloud - should appear in your "Domains" list.»
It would benefit the reader to know why she clicks and where it goes, before actually starting. Also, it helps planning ahead: «should I start now if I can’t even decide for a subdomain? (because I want to talk about it with family, because I want to sleep on it, whatever)».
Great job anyway, it’s great and helpful as it is.