Fix #120 - Add 404 pages

To get proper 404 pages for the docs, I had to change some things, and then I changed a lot of the docs file loading and building... :anguished: 
The default file is loaded here: https://github.com/mootools/website/pull/139/files#diff-ae48f1c21623e6247da03c9c62cb8eb4R39