Ontology docs weren't in a much good shape, besides many ontologies
being seriously underdocumented (something which should improve
separately), the generated docs were little more than a data dump,
and the diagrams shown were broken, confusing, or both. This all
amounts to quite counter-productive developer docs.

So the ontology docs have been refurbished, the per-ontology
descriptions are still useful, but have been stripped of all images,
and the docs overall are now completely class-centric, per
rdfs:Resource subclass we now get:

- Ascii diagram of its local hierarchy, up to all its ancestors and
  down to all its direct children.
- All properties that affect the specific class. This is notably
  more intuitive now as there's properties defined on one ontology
  that are in the domain of classes in another ontology, something
  which you couldn't get at a glance in the previous docs
- It clearly states which properties supersede which superproperties,
  which again makes it easier if those apply for the class at hand.

The result feels quite neater, and will indeed be more resembling
to other gtk-doc generated API docs.