Use Jinja the way it is supposed to be used

Old code created the final layout in header.html and footer.html, then
loaded each .html file as a Jinja template for further post-processing.

We now use an empty footer.html and an almost empty header.html
(just contains some vars to capture Doxygen variables), then load a real
Jinja template once and process all files with it.

This is less error prone and much faster (postprocessing kdelibs4support now
takes 1 second instead of 18 before)