When answering a question here on Ask Ubuntu I sometimes would like to link to a wiki page explaining some topic in more detail, say, the ALSA sound system.
For German speaking users there's the very useful wiki.ubuntuusers.de, but what about an English equivalent? wiki.ubuntu.com comes to mind, but it doesn't even have a page for ALSA, or .profile, or Dolphin, or KDE – you can't be serious!
When looking for information just for myself, I often end up using the excellent Arch Linux wiki, but of course it's weird to link to the Arch Linux wiki to explain Ubuntu stuff.
What did I miss?
Unfortunately your observation that the https://wiki.ubuntu.com page is not nearly complete and often outdated is correct. Some pages there are useful, but more often you will be rather disappointed than satisfied.
There is also the official documentation on https://help.ubuntu.com/ though. You can link to pages of its HTML version.
More info about the differences between the Ubuntu Wiki, Community Wiki and Ubuntu Help pages can be found at What is the difference between the *.ubuntu.com wikis?
The Arch Wiki is amazing in its size and detail, and quite often there is few or no difference between Arch and Ubuntu (except mainly package management stuff, etc), so linking an Arch Wiki article can be perfectly fine. Just make sure you verify that the important parts are valid for both systems before posting.
Other than that, popular posts here on Ask Ubuntu (searching for keywords and sorting the results by vote score or a site-search on e.g. Duck Duck Go or Google will help you to find appropriate posts) or e.g. https://ubuntuforums.org can also serve as valuable reference.
Sometimes actually even Wikipedia does the job.