Add siunitx.js

This helps format SI units in MathJax.

_includes/mathjax.html

 <script type="text/x-mathjax-config">
+  MathJax.Ajax.config.path['siunitx'] = "{{ "/assets/js/" | real_relative_url }}";
   MathJax.Hub.Config({
     extensions: [
       "MathMenu.js",
       "MathZoom.js",
       "AssistiveMML.js",
-      "a11y/accessibility-menu.js"
+      "a11y/accessibility-menu.js",
+      "[siunitx]/siunitx.js",
     ],
     jax: ["input/TeX", "output/CommonHTML"],
     TeX: {
@@ -13,6 +15,7 @@
         "AMSsymbols.js",
         "noErrors.js",
         "noUndefined.js",
+        "[siunitx]/siunitx.js"
       ],
       equationNumbers: {
         autoNumber: "AMS"