MathJax v3
You can customise Just the Docs sites to support MathJax, as explained in the configuration suggestions. Pages then render \(\mathrm{\LaTeX}\) code in kramdown math blocks using MathJax.
For example:
\[\begin{aligned} & \phi(x,y) = \phi \left(\sum_{i=1}^n x_ie_i, \sum_{j=1}^n y_je_j \right) = \sum_{i=1}^n \sum_{j=1}^n x_i y_j \phi(e_i, e_j) = \\ & (x_1, \ldots, x_n) \left( \begin{array}{ccc} \phi(e_1, e_1) & \cdots & \phi(e_1, e_n) \\ \vdots & \ddots & \vdots \\ \phi(e_n, e_1) & \cdots & \phi(e_n, e_n) \end{array} \right) \left( \begin{array}{c} y_1 \\ \vdots \\ y_n \end{array} \right) \end{aligned}\]To see the \(\mathrm{\LaTeX}\) source of the formula, right-click anywhere on it.
Some further tests illustrate the use of math in Markdown.