readded hugo-book theme files.

themes/hugo-book/assets/clipboard.js

@@ -0,0 +1,12 @@
+(function () {
+  document.querySelectorAll("pre:has(code)").forEach(code => {
+    code.addEventListener("click", code.focus);
+    code.addEventListener("copy", function (event) {
+      event.preventDefault();
+      if (navigator.clipboard) {
+        const content = window.getSelection().toString() || code.textContent;
+        navigator.clipboard.writeText(content);
+      }
+    });
+  });
+})();