update html documentation building

There was a bunch of cmake code and a shell script to change the links that KDocTools generates in the html documentation (running sed s+help:/common+./common+g and sed s+help:/kdoctools6-common+./common+g on the html files). In my opinion that would be better changed in KDocTools, rather than a bunch of custom stuff here. I see now that KDocTools has a stylesheet that works for this without editing the html.