We use the Saxon-HE (home edition) version 9 . It can be downloaded from http://saxon.sourceforge.net/
This is explained in detail elsewhere.
$ cp2k.popt --xml
This generates the file cp2k_input.xml
which contains the entire input description.
$ cd cp2k/tools/manual $ java -jar saxon9he.jar -o:index.html cp2k_input.xml cp2k_input.xslcp2k.popt
This will create the actual HTML files, which we make available at http://manual.cp2k.org/