[WIP] Move union directive to be class-like

[jakobandersen]

The union directive is currently classified as an "item", similar to typedefs, enums, and enumerators. However, formally it is a class type (https://en.cppreference.com/w/cpp/language/type), and in terms of documentation I think it should be handled as classes are. See also #698.

The HTML for the Breathe docs are unaffected by this change.

Fixes #793?

TODOs:

• More extensive testing. Directive refactoring #698 mentions something about namespacing that I don't remember the details of, but should be double checked.'
• Docugmentation update?
• Changelog entry.

[dmg0345]

Any chance this could be merged?