feat(deps): move MathJax configuration to a separate file (#1670)

Move `MathJax` configuration to file `assets/js/data/mathjax.js` will help add extensions.

assets/js/data/mathjax.js

@@ -0,0 +1,25 @@
+---
+layout: compress
+# WARNING: Don't use '//' to comment out code, use '{% comment %}' and '{% endcomment %}' instead.
+---
+
+{%- comment -%}
+  See: <https://docs.mathjax.org/en/latest/options/input/tex.html#tex-options>
+{%- endcomment -%}
+
+MathJax = {
+  tex: {
+    {%- comment -%} start/end delimiter pairs for in-line math {%- endcomment -%}
+    inlineMath: [
+      ['$', '$'],
+      ['\\(', '\\)']
+    ],
+    {%- comment -%} start/end delimiter pairs for display math {%- endcomment -%}
+    displayMath: [
+      ['$$', '$$'],
+      ['\\[', '\\]']
+    ],
+    {%- comment -%} equation numbering {%- endcomment -%}
+    tags: 'ams'
+  }
+};