docs: update deeplink to digitalocean marketplace image on homepage
docs: update deeplink to digitalocean marketplace image on homepage

The images have changed location. Also add a referral code to make it possible to run Dokku tests without breaking the bank.
claytonbrown/dokku,dokku/dokku,claytonbrown/dokku,progrium/dokku,claytonbrown/dokku,progrium/dokku,progrium/dokku,dokku/dokku,claytonbrown/dokku,dokku/dokku,claytonbrown/dokku,dokku/dokku,claytonbrown/dokku,progrium/dokku,dokku/dokku,dokku/dokku,claytonbrown/dokku,progrium/dokku,dokku/dokku,progrium/dokku,claytonbrown/dokku,progrium/dokku,dokku/dokku,progrium/dokku