Not really an alternative, since C++ projects often use Breathe to "transpile" Doxygen to Sphinx. I even contributed a few patches to Doxygen from which the Breathe/Sphinx project directly benefited, but after trying it out a bit and hitting a few walls, I realized I would achieve better flexibility by having my own frontend.

Especially because (if I can dare to say) I ended up with a much better search than what Sphinx (or Doxygen) has -- try it out: https://doc.magnum.graphics/magnum/?q=max#search ;)