Export math to intersphinx #9483
Description
Is your feature request related to a problem? Please describe.
It would be nice to be able to reference equations available in other Sphinx projects, in the same way we reference documents, labels, or code objects.
Describe the solution you'd like
Have :math: and .. math:: objects available in the intersphinx objects.inv.
Describe alternatives you've considered
To my knowledge, the only alternative is to hardcode a link to the closest heading the user wants.
Additional context
Comes from #9459 (comment).