Doc: rename index.html as extending-index.html

Remove the \contextpage commands, as they are only used
in some of the files.

Task-number: QTBUG-31106
Change-Id: Iacbf15a33ab6cb426c2fbfb6fbed03310095f2ba
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>