Move docs/zh back to docs.

Move docs/overrides to overrides/.
Other fine tunes.

overrides/javascripts/katex.js

@@ -0,0 +1,10 @@
+document$.subscribe(({ body }) => {
+  renderMathInElement(body, {
+    delimiters: [
+      { left: "$$", right: "$$", display: true },
+      { left: "$", right: "$", display: false },
+      { left: "\\(", right: "\\)", display: false },
+      { left: "\\[", right: "\\]", display: true },
+    ],
+  });
+});

overrides/javascripts/mathjax.js

@@ -0,0 +1,16 @@
+window.MathJax = {
+  tex: {
+    inlineMath: [["\\(", "\\)"]],
+    displayMath: [["\\[", "\\]"]],
+    processEscapes: true,
+    processEnvironments: true,
+  },
+  options: {
+    ignoreHtmlClass: ".*|",
+    processHtmlClass: "arithmatex",
+  },
+};
+
+document$.subscribe(() => {
+  MathJax.typesetPromise();
+});