I highly suggest WordPress, heck it's good enough for The Huffington Post and The New York Times. If you need assistance, contact me.
Yup, WordPress for sure. We've been running on that for about a decade. It's very easy to use and extremely flexible. Tons of themes already available or you can make your own, of course, but honestly that is a waste, just buy one or use a free one. It is backed by MySQL and can scale like crazy. And it is very easy to front it with a CDN like CloudFlare to make things even faster and safer.
Run it on a LAMP stack for best results, IMHO. CentOS 6.5 is my recommendation. Everything you need is available via YUM (except WP itself, of course, although I hope that they include that in the future) and works together immediately.
The idea of creating the document to help you build out your documentation in theory should be a one time thing... As the management side of things I would say that there is hardly ever a case where too much documentation is an issue. All documentation should be done on the theory of "what if I get hit by a bus today?" can someone else step into my position seamlessly and the client not see any lag?
Too much documentation, though, can result in people being unable to find what is needed and the time needed to maintain it can become a point of inefficiency. And the more that there is, the more likely that it will go out of date and become a negative rather than a positive. Only good documentation is useful, and the more documentation you have, the higher chances that some of it will not be maintained.
We had an old guy that would "document" everything. He had tons of binders full of stuff and his home directory was huge with "documentation." His documentation was he would make a tiny note and then run a command and copy the output. That's pretty much all his notes were spanning back to ~2004-2005. He left and we ended up throwing it all away because we couldn't find anything useful at all.