There is a fair amount of content on the CentOS wiki but, as you've noticed, some of it is
extremely outdated or was written for older CentOS versions. One page on there that I came across recently was almost 10 years old!
Depending on what the topic is, the age of an article may not be all that relevant -- the important thing is to make sure that it's aimed at the version of CentOS that you're running. Some of those old wiki pages were written for CentOS 4 and that's been EOL for over six years!
If you have run into problems or have some specific questions, the CentOS forums can be a great resource. There seems to be some pretty knowledgeable folks there and the moderators will, at the least, ensure that no incorrect or outdated information is given out.
Finally, remember that Red Hat publishes a tremendous amount of technical documentation for RHEL 7 [0] which (with a few exceptions) also applies directly to CentOS 7. The PDF versions of their "Installation Guide" [1] and "System Administrator's Guide" [2] for RHEL 7 weigh in at 489 and 602 pages, respectively.
At times, you may come across knowledge base articles or other documentation on their "customer portal" (access.redhat.com) [3] but that you can't see because it requires an account. Fortunately, they have a (free!) "Red Hat Enterprise Linux Developer Program" [4] that you can sign up for which will give you access to that "customer portal" in addition to providing you with a valid license to run RHEL (as well as a lot of their other stuff) for development/testing purposes. See the original announcement [5] and/or press release [6] for more information.
[0]: https://access.redhat.com/documentation/en-us/red_hat_enterp...
[1]: https://access.redhat.com/documentation/en-us/red_hat_enterp...
[2]: https://access.redhat.com/documentation/en-us/red_hat_enterp...
[3]: https://access.redhat.com/documentation/en-us/
[4]: https://access.redhat.com/products/Red_Hat_Enterprise_Linux/...
[5]: https://developers.redhat.com/blog/2016/03/31/no-cost-rhel-d...
[6]: https://www.redhat.com/en/about/press-releases/red-hat-expan...