Generate sitemap from template (#456)

diff --git a/netbeans.apache.org/globals.yml b/netbeans.apache.org/globals.yml
index 5b68802..cd91dc0 100644
--- a/netbeans.apache.org/globals.yml
+++ b/netbeans.apache.org/globals.yml
@@ -20,7 +20,7 @@
 
 render:
     tags: false
-    sitemap: false
+    sitemap: true
     index: false
     archive: false
     feed: false
@@ -42,6 +42,8 @@
         extension: .asciidoc
     raw:
         file: raw.gsp
+    sitemap:
+        file: sitemap.gsp
 
 asciidoctor:
     attributes:
diff --git a/netbeans.apache.org/src/content/templates/sitemap.gsp b/netbeans.apache.org/src/content/templates/sitemap.gsp
new file mode 100644
index 0000000..0fc23c9
--- /dev/null
+++ b/netbeans.apache.org/src/content/templates/sitemap.gsp
@@ -0,0 +1,28 @@
+<%/*
+
+    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.
+
+*/%><?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+<%published_content.each {content -> %>
+    <url>
+        <loc>${config.site_host}/${content.uri}</loc>
+        <lastmod>${content.date.format("yyyy-MM-dd")}</lastmod>
+    </url>
+<%}%>
+</urlset>