readded hugo-book theme files.

themes/hugo-book/layouts/_partials/docs/search.html

@@ -0,0 +1,10 @@
+{{ if default true .Site.Params.BookSearch }}
+<link href="/pagefind/pagefind-ui.css" rel="stylesheet">
+<script src="/pagefind/pagefind-ui.js"></script>
+<div id="search"></div>
+<script>
+    window.addEventListener('DOMContentLoaded', (event) => {
+        new PagefindUI({ element: "#search", showSubResults: true });
+    });
+</script>
+{{ end }}