The old docs site was great, well organised and a good quick reference guide. I feel that approach should be maintained as much as possible. Frameworks are a scary prospect for fledgling developers and Codeigniters good documentation is what lead me to it many years ago and to what keeps me using it over newer alternatives.
Essentially keep it clean and simple. The updated PHP site isn't a bad example of how to do it well - if the layout HAS to change.