RFR: JDK-8180426 Use standard css file for new docs bundle index.html page

[Magnus Ihse Bursie]

In JDK-8180208, a new top-level index.html for the entire docs image was 
created.

This should use the common jdk-default.css file, which should move out 
of the specs directory.

Here is an example how the generated page looks like:
http://cr.openjdk.java.net/~ihse/demo-docs-index-with-default-css/

Bug: https://bugs.openjdk.java.net/browse/JDK-8180426
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8180426-use-css-for-bundle-index-page/webrev.01

/Magnus