Upgrade MathJax library
There is code in config.js (MathAvailable) / dokieli.js (initMath, MathJax) relying on MathJax v2.7.
Upgrade to MathJax v3 (or later).
https://docs.mathjax.org/en/latest/upgrading/v2.html
https://mathjax.github.io/MathJax-demos-web/convert-configuration/convert-configuration.html
docs currently loads https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.2/MathJax.js?config=TeX-MML-AM_HTMLorMML and there is example math code in the document that can be used to test.
See also math feature available via a ProseMirror schema/plugin, e.g., https://github.com/benrbray/prosemirror-math (or possibly ProseMirror using MathJax renderer: https://github.com/benrbray/prosemirror-math/issues/4 ). There may be other PM plugin.
Either way, It'd be good to have one that supports MathML output so that can be part of the document when stored.