The one shown in the animation on the repo page appears to be typical for a markdown cell in any Jupyter page. Maybe they have since added a way to make it easier? I couldn’t see such a thing in the Binder launched version at this time.
In the animation, it looks like the following code is used.
$s= \sqrt{\frac{1}{N-1} \sum_{i=1}^N (x_i - \overline{x})^2}$
Followed by hitting return after the closing dollar sign to trigger rendering it.
For inline, you want to flank the latex by dollar signs. For displayed formulas (at the center), you’d use double dollar signs to flank it. You may find this quick reference helpful.
In the rich text editor, you then have to click directly on the formula to edit it.
(I note that if you go back and forth between markdown and raw in the rich text editor, the equations can become ‘corrupt’ by doubling up the backslashes to escape the code.)