Is your feature request related to a problem? Please describe.

Currently in a documentation project we have many pages, of which only a very small subset needs mathjax. However, enabling the sphinx.ext.mathjax extension causes mathjax.js to be included in every page. This is a bit unfortunate since mathjax is huge.

Describe the solution you'd like

Mathjax.js is included in only the pages which need it, and not every page.

Describe alternatives you've considered

Current system works, but makes all pages that don't need mathjax bigger and slower than they need to.

Additional context

I mentioned this as a comment in #5497 , where it was suggested that I open a new issue.