perf: enable equation numbering in MathJax (#1520)

If you place your equations inside $$\begin{equation}...\end{equation}$$
you will get MathJax equation numbering. As described here:
https://docs.mathjax.org/en/latest/input/tex/eqnumbers.html

BREAKING CHANGES: if you used \begin{equation} inside $$ you will
get equation numbering which was not previously present.

_posts/2019-08-08-text-and-typography.md

@@ -134,7 +134,14 @@ fi;
 
 The mathematics powered by [**MathJax**](https://www.mathjax.org/):
 
-$$ \sum_{n=1}^\infty 1/n^2 = \frac{\pi^2}{6} $$
+$$
+\begin{equation}
+  \sum_{n=1}^\infty 1/n^2 = \frac{\pi^2}{6}
+  \label{eq:series}
+\end{equation}
+$$
+
+We can reference the equation as \eqref{eq:series}.
 
 When $a \ne 0$, there are two solutions to $ax^2 + bx + c = 0$ and they are