Implemented page categorization functionality
* expanded "02_data/pages.xsl" transformation to include pages in subfolders
* "03_meta/categories.xsl" transformation generates a categorized view of all pages simmilar to the one provided for tags by "03_meta/tags.xsl"
* "99_result/category/category.xsl" transformation generates category overview pages
* added basic project related pages inside the "projects" category