The \indexpage command is no longer used. This update removes it
from qdoc and from the qdoc manual.

Change-Id: I7ac0cdc2d7fffc91dda01024740478e7da5570f0
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>