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.

_includes/js-selector.html

@@ -85,7 +85,9 @@
         displayMath: [
           ['$$', '$$'],
           ['\\[', '\\]']
-        ]
+        ],
+        /* equation numbering */
+        tags: 'ams'
       }
     };
   </script>