Doc: Reverted name of Qt WebKit and landing page filename.

-qtwebkit-index.html and "Qt WebKit" are the correct names for the
module according to the writing guidelines
-index.html will collide with the main Qt 5 index.html page

Change-Id: Ic07ae422b85766090b1e3c09b34100293a972bfb
Reviewed-by: Martin Smith <martin.smith@digia.com>