That's kind of a loaded question; everyone seems to have their preference when it comes to using a pre-built CMS (Content Management System). A couple of the more popular ones ar Drupal and Joomla which are pretty similar, and have tons of user developed addon modules to expand on what the basic framework does in countless ways.

Some have strengths in areas that others have weaknesses so the best thing to do is install a handful of different ones and see which seems to feel best to you. I know that Drupal doesn't really fare too well in the built-in chat room area, plus even when I've installed a few different ones, people rarely used them. If forced to in a work / company situation though, they might.

As far as documentation goes, it might be better to use something like a Wiki CMS, which is basically exactly what they were created for. Other CMS's can do it too, but possibly not quite as well.

Take a look here at a big list of CMS's available.