chore: Add MathJax support to head.html

Author: jamiesun

_includes/head.html

@@ -66,4 +66,5 @@
 
   {% include fonts.html %}
   {% include google_analytics.html %}
+  {% include mathjax.html %}
 </head>

_includes/mathjax.html

@@ -0,0 +1,19 @@
+<script>
+  MathJax = {
+    tex: {
+      inlineMath: [
+        ["$", "$"],
+        ["\\(", "\\)"],
+      ],
+    },
+    svg: {
+      fontCache: "global",
+    },
+  };
+</script>
+<script
+  type="text/javascript"
+  id="MathJax-script"
+  async
+  src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-svg.js"
+></script>