[Doc] Reduce browser memory usage for docs (#3870)

Signed-off-by: David Chen <530634352@qq.com>

docs/mkdocs/javascript/mathjax.js

@@ -1,20 +1,44 @@
-// Enables MathJax rendering
+// Enables MathJax rendering only on pages that contain math.
 window.MathJax = {
-    tex: {
-      inlineMath: [["\\(", "\\)"]],
-      displayMath: [["\\[", "\\]"]],
-      processEscapes: true,
-      processEnvironments: true
-    },
-    options: {
-      ignoreHtmlClass: ".*|",
-      processHtmlClass: "arithmatex"
-    }
-  };
+  tex: {
+    inlineMath: [["\\(", "\\)"]],
+    displayMath: [["\\[", "\\]"]],
+    processEscapes: true,
+    processEnvironments: true,
+  },
+  options: {
+    ignoreHtmlClass: ".*|",
+    processHtmlClass: "arithmatex",
+  },
+};
 
-  document$.subscribe(() => {
-    MathJax.startup.output.clearCache()
-    MathJax.typesetClear()
-    MathJax.texReset()
-    MathJax.typesetPromise()
-  })
+let mathJaxLoading;
+
+function loadMathJax() {
+  if (window.MathJax.typesetPromise) {
+    return Promise.resolve();
+  }
+  if (!mathJaxLoading) {
+    mathJaxLoading = new Promise((resolve, reject) => {
+      const script = document.createElement("script");
+      script.src = "https://unpkg.com/mathjax@3.2.2/es5/tex-mml-chtml.js";
+      script.async = true;
+      script.onload = resolve;
+      script.onerror = reject;
+      document.head.appendChild(script);
+    });
+  }
+  return mathJaxLoading;
+}
+
+document$.subscribe(() => {
+  if (!document.querySelector(".arithmatex")) {
+    return;
+  }
+  loadMathJax().then(() => {
+    MathJax.startup.output.clearCache();
+    MathJax.typesetClear();
+    MathJax.texReset();
+    MathJax.typesetPromise();
+  });
+});