documentation: add license headers to various files

Documentation/_templates/layout.html

@@ -1,3 +1,22 @@
+<!--
+ Documentation/_templates/layout.html
+
+ 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.
+-->
+
 {% extends "!layout.html" %}
 {% block sidebartitle %}
   {% if logo and theme_logo_only %}