See the discussion beginning here:
http://mail.openjdk.java.net/pipermail/javadoc-dev/2018-June/000514.html

A side-effect of JDK-8196202 is that there is no longer any need to generate the overview-summary.html file, since the content that it used to contain is now in index.html.

This causes unexpected "404 Not Found" for anyone who has bookmarked the overview-summary.html file.

The proposed solution is to generate a "placeholder" file that is a redirect to index.html.