Upgrade from MathJax v2.7 to v3.

The first part of the config is easy to upgrade using this tool:
http://docs.mathjax.org/en/latest/upgrading/v2.html#upgrading-from-v2-to-v3

The second part, I added to implement the "\psmallmatrix" environment
from the "mathtools" package (which MathJax doesn't support), because I used
it in the blog posts
https://quuxplusone.github.io/blog/2018/08/05/quantum-circuits/
https://quuxplusone.github.io/blog/2018/08/05/mathjax-in-jekyll/

This solution is due to Davide Cervone:
https://groups.google.com/g/mathjax-users/c/lWj4CeN70oY/m/Prox7m85AgAJ
https://stackoverflow.com/questions/63428119/

_includes/mathjax.html

@@ -1,36 +1,23 @@
-<script type="text/x-mathjax-config">
-  MathJax.Hub.Config({
-    extensions: [
-      "MathMenu.js",
-      "MathZoom.js",
-      "AssistiveMML.js",
-      "a11y/accessibility-menu.js"
-    ],
-    jax: ["input/TeX", "output/CommonHTML"],
-    TeX: {
-      extensions: [
-        "AMSmath.js",
-        "AMSsymbols.js",
-        "noErrors.js",
-        "noUndefined.js",
-      ]
+<script type="text/javascript">
+window.MathJax = {
+  tex: {
+    packages: ['base', 'ams']
+  },
+  loader: {
+    load: ['ui/menu', '[tex]/ams']
+  },
+  startup: {
+    ready() {
+      MathJax.startup.defaultReady();
+      const Macro = MathJax._.input.tex.Symbol.Macro;
+      const MapHandler = MathJax._.input.tex.MapHandler.MapHandler;
+      const Array = MathJax._.input.tex.ams.AmsMethods.default.Array;
+      const env = new Macro('psmallmatrix', Array, [null, '(', ')', 'c', '.333em', '.2em', 'S', 1]);
+      MapHandler.getMap('AMSmath-environment').add('psmallmatrix', env);
     }
-  });
-  MathJax.Hub.Register.StartupHook("TeX Jax Ready",function () {
-    var TEX = MathJax.InputJax.TeX;
-    var COLS = function (W) {
-      var WW = [];
-      for (var i = 0, m = W.length; i < m; i++)
-        {WW[i] = TEX.Parse.prototype.Em(W[i])}
-      return WW.join(" ");
-    };
-    TEX.Definitions.Add({
-      environment: {
-        psmallmatrix: ['Array',null,'(',')','c',COLS([1/3]),".2em",'S',1],
-      }
-    });
-  });
+  }
+};
 </script>
-<script type="text/javascript" async
-  src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/latest.js">
+<script type="text/javascript" id="MathJax-script" async
+  src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-chtml.js">
 </script>