RFR: JDK-8204303: Add redirect for overview-summary.html

[Jonathan Gibbons]

The recent change to update javadoc to not generate frames by default 
means that the overview-summary.html file is no longer generated: the 
content that was previously in that file is now in the top-level 
index.html file.

This means that any saved bookmarks to the overview-summary.html file 
are invalid.

The fix is to generate a dummy overview-summary.html that simply 
redirects to the top-level index.html file. The file is only generated 
when it would otherwise have been generated in frames mode. The file may 
eventually be removed in an unspecified future release, after suitable 
explicit warnings have been made.

The change leverages the existing code in IndexRedirectWriter.java, by 
generalizing it to create redirect files from any file to any other file 
in the doc bundle.

JBS: https://bugs.openjdk.java.net/browse/JDK-8204303
Webrev: http://cr.openjdk.java.net/~jjg/8204303/webrev.00/index.html

-- Jon