qdoc: Add support for class diagrams

As we parse the header files with libclang, we could also store relevant information for generating class diagrams to display on reference pages. Options include at least

• graphviz
• mermaid (JavaScript library)
• custom HTML + CSS

If possible, try to avoid an image + map approach, as it doesn't serve screen readers too well. Also, a solution that would extend also to QML type reference would be good.