kernel-doc: generate main index page when building 'htmldocs'

A patch for kernel-doc that enables the generation of a global, TOC-like
index.html page after building 'htmldocs'

Signed-off-by: Borislav Petkov <bbpetkov@yahoo.de>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>