Request Review JDK-8182029: Make the top-level docs index.html to a HTML-level redirect to the API overview page

[Mandy Chung]

http://cr.openjdk.java.net/~mchung/jdk9/webrevs/8182029/webrev.00/

JBS issue:
  https://bugs.openjdk.java.net/browse/JDK-8182029

This patch replaces the top-level docs index.html to a HTML-level redirect to 
the API overview page.  We can come back to this page later in a future release 
to convey grouping information.

Mandy