chore: import upstream snapshot with attribution

ui/template/404.html

@@ -0,0 +1,29 @@
+<!--
+
+    Licensed to the Apache Software Foundation (ASF) under one
+    or more contributor license agreements.  See the NOTICE file
+    distributed with this work for additional information
+    regarding copyright ownership.  The ASF licenses this file
+    to you under the Apache License, Version 2.0 (the
+    "License"); you may not use this file except in compliance
+    with the License.  You may obtain a copy of the License at
+
+      http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing,
+    software distributed under the License is distributed on an
+    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+    KIND, either express or implied.  See the License for the
+    specific language governing permissions and limitations
+    under the License.
+
+-->
+{{template "header" . }}
+            <div class="d-flex flex-column justify-content-center align-items-center page-wrap container">
+                <div class="mb-4 text-secondary" style="font-size: 120px; line-height: 1.2;">(=‘x‘=)</div>
+                <div class="text-center mb-4">Unfortunately, this page doesn't exist.</div>
+                <div class="text-center"><a role="button" tabindex="0" href="{{$.baseURL}}/" class="btn btn-link">Back to
+                        homepage</a></div>
+            </div>
+
+{{template "footer" .}}