Fix: Treat partial class template specializations correctly

[fknorr]

cldoc currently does not generate nodes for partial template class specializations. It still tries to process its children, which receive the containing namespace as parent (fallback). This causes problems elsewhere, e.g. if the child is a method, it expects its parent to be a class.

This PR models partial template class specializations as class templates, fixing this issue. The change requires Node types to map to multiple kinds.