Reference an ASF page as the main place to find third-party packages

Some comments and docs like https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/DataSource.scala#L148-L151 say to go to spark-packages.org, but since this is a package index maintained by a third party, it would be better to reference an ASF page that we can keep updated and own the URL for.