doc/sphinx/doxygen.ini.in: Remove CLASS_DIAGRAM

CLASS_DIAGRAM has been obsolete in newer version of doxygen, and
it's enabled if HAVE_DOT and CLASS_GRAPH are set.

This increase DOT_GRAPH_MAX_NODES to avoid dot complaning,
and include dot/graphviz for doxygen.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>