The documentation of the ontologies is build as follows:

- Each ontology comes with a docbook file which contains the authors and copyright info and any documentation and howtos that might be of interest.
- Each ontology docbook contains an xinclude like "<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="nie-reference.docbook"/>"
- build-ontology-reference.py creates one reference file like "nie-reference.docbook" for each ontology in the working dir (TODO: add a parameter to change the target
  folder)
- All docbooks are processed with the modififed XSLT stylesheet: "xsltproc --output nie.html --xinclude sdo-docbook.xls nie.docbook" to generate the final
  web pages
