I think the best question is when after asking in you find the answer :) So I found a "workaround" at least I think it is a workaround and not a solution. For me that is perfect:
I've added "categories" to docpad.coffee file
templateData: site: categories: ['Tutorials','General']
Now I will always update this array with categories that should be used in meta info of each markdown doc
My loop looks like this now....and works!!!
<% for category in @site.categories : %> <li><h2><%- category %></h2> <ul> <%for docu in @getFilesAtPath("document-repository",[{filename: -1}]).findAll({category:category}).toJSON():%> <li><a href="#<%=docu.url%>"><%=docu.title%></a></li> <% end %> </ul> </li> <% end %>