Add Mathjax support

[asoplata]

Current building does not seem to provide support for full TeX mathematical expression. This can be seen if one puts the following sentence (from here https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/writing-mathematical-expressions#writing-inline-expressions ) into a markdown file and tries to build it:

This sentence uses `$` delimiters to show math inline: $\sqrt{3x-1}+(1+x)^2$

If I include that, I get the following output:

make build
python build.py
[WARNING] Could not convert TeX math \sqrt{3x-1}+(1+x)^2, rendering as TeX
[WARNING] Could not convert TeX math -\sqrt{2}/2, rendering as TeX

And, despite its claim to render as TeX, it does not perform any fancy rendering:

This is similar to the earlier version that also was not rendered in the the "Under the hood" section on currents: https://jonescompneurolab.github.io/hnn-under_the_hood/06_primary-electrical-currents/06_primary-electrical-currents