Clarify `index` Gradle extension docs that it configures all generated pages, not just index.html

[okarmazin]

The KDoc of IndexBlock and its properties only talks about "index.html", but the configuration therein affects all generated HTML pages. The "index" naming should probably be changed to something more fitting.

[bitspittle]

Great point. We discussed this issue and actually you might have helped us realize we have a bigger change in mind to fix this than just renaming.

I'll try to put together a doc proposal to capture some thoughts and will paste that link here.