wrong namespace/folder :)
namespace Anchorbrands\Bundle\LandingpageBundle\Entity;
should be
namespace Anchorbrands\Bundle\LandingpageBundle\Document;
The documents usually live in the Document folder.
Entity folder/namespace is for ORM not ODM.
Have a look at the documentation here.
Wrong command
for mongodb / doctrine odm you need to use ...
app/console doctrine:mongodb:generate:documents AnchorbrandsLandingpageBundle
... while doctrine:generate:entities
is for doctrine orm.