Add header to clarify that book-v3 is legacy documentation

docs/book/theme/css/custom.css

@@ -1,3 +1,30 @@
 .menu-title img {
     vertical-align: bottom;
 }
+
+#notice-bar {
+    background: var(--bg);
+    padding: 1em;
+    margin-left: calc(-1 * var(--page-padding));
+    margin-right: calc(-1 * var(--page-padding));
+    margin-bottom: 1em;
+
+    border-bottom: 1px solid var(--table-border-color);
+    box-shadow: 0 1px 5px 0 var(--table-border-color);
+}
+
+#notice-bar * {
+    color: var(--fg);
+}
+
+#notice-bar a {
+    text-decoration: none;
+}
+
+#notice-bar a:not(.header) {
+    color: var(--links);
+}
+
+#notice-bar h2 {
+    margin-top: 0;
+}

docs/book/theme/header.hbs

@@ -0,0 +1,3 @@
+<header id="notice-bar">
+    <h2>Viewing legacy documentation for Kubebuilder, check out the <a href="https://book.kubebuilder.io">latest</a> documentation instead.</h2>
+</header>