Add category pages and tags.

This change adds a category for all pages generated by the
GraphViz extension.  There is one "root" category for GraphViz
and a subcategory for each renderer (dot, neato, fdp, sfdp,
circo, twopi and mscgen).  This makes it easy to find graph
images created by the extension.

Change-Id: I942f703614165d7cadcf501b1312139e265bf5f0