replace qt-project.org/doc with doc.qt.io
also rename macosx-building.html -> osx-building.html

Change-Id: I5b4aaa187562b653407f1f99947df748c3240554
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>