chore: add useKatex option

gatsby/on-render-body.js

@@ -0,0 +1,22 @@
+'use strict';
+
+const React = require('react');
+const siteConfig = require('../config.js');
+
+// eslint-disable-next-line import/no-webpack-loader-syntax, import/no-unresolved
+const katexStylesheet = require('!css-loader!../static/css/katex/katex.min.css');
+
+const onRenderBody = ({ setHeadComponents }) => {
+  const { useKatex } = siteConfig;
+
+  if (useKatex) {
+    setHeadComponents([
+      React.createElement('style', {
+        key: 'katex-inline-stylesheet',
+        dangerouslySetInnerHTML: { __html: katexStylesheet.toString() }
+      })
+    ]);
+  }
+};
+
+module.exports = onRenderBody;