Update Downloads page to adhere to the ASF policy

1. Remove links to dist.apache.org
2. Use closer.lua for source artifacts
3. Add link to the official KEYS file
4. Add link to instructions for verifying releases

ASF policy for download pages: https://infra.apache.org/release-download-pages.html

website/releases/downloads.mdx

@@ -3,7 +3,12 @@ title: Downloads
 sidebar_position: 1
 ---
 
+Apache XTable™ (incubating) releases are available under the Apache License, Version 2.0. See the NOTICE file contained in each release artifact for applicable copyright attribution notices.
+
+To ensure that you have downloaded the true release, you should [verify](https://www.apache.org/info/verification.html) the integrity of the files using the signatures and checksums available in this page. All releases are signed using the [Apache XTable KEYS file](https://downloads.apache.org/incubator/xtable/KEYS).
+
 ### Release 0.1.0-incubating
-* Source Release: [Apache XTable 0.1.0-incubating Source Release](https://dist.apache.org/repos/dist/release/incubator/xtable/0.1.0-incubating/apache-xtable-0.1.0-incubating.src.tgz) ([asc](https://dist.apache.org/repos/dist/release/incubator/xtable/0.1.0-incubating/apache-xtable-0.1.0-incubating.src.tgz.asc), [sha512](https://dist.apache.org/repos/dist/release/incubator/xtable/0.1.0-incubating/apache-xtable-0.1.0-incubating.src.tgz.sha512))
+* Source Release: [Apache XTable 0.1.0-incubating Source Release](https://www.apache.org/dyn/closer.lua/incubator/xtable/0.1.0-incubating/apache-xtable-0.1.0-incubating.src.tgz) ([asc](https://downloads.apache.org/incubator/xtable/0.1.0-incubating/apache-xtable-0.1.0-incubating.src.tgz.asc), [sha512](https://downloads.apache.org/incubator/xtable/0.1.0-incubating/apache-xtable-0.1.0-incubating.src.tgz.sha512))
 * Release Notes: [Release Notes 0.1.0-incubating](release-0.1.0-incubating.mdx)
-* Maven Search: [Apache XTable 0.1.0-incubating](https://central.sonatype.com/search?q=g:org.apache.xtable%20v:0.1.0-incubating&smo=true)
+* Maven Search: [Apache XTable 0.1.0-incubating](https://central.sonatype.com/search?q=g:org.apache.xtable%20v:0.1.0-incubating&smo=true)
+