Support Tagfiles

[Nerixyz]

Doxygen has the option to specify tagfiles which link to external documentation. One example is Qt which hosts its tagfiles https://doc.qt.io/qt-6/qtcore.tags (and qtgui, qtwidgets, and so on). Currently, the generator fails to resolve these references.

[JakubAndrysek]

Thank you for sharing this information. MkDoxy currently does not support all tags. Adding more parsers and processing would be excellent, but I currently don't have time for that.

Feel free to implement it.