Update the spree preferences guide

`site_name`, `site_url`, `default_meta_description`, `default_meta_keywords`, and `default_seo_title` are deprecated in Spree 2.3.0. Its probably a good idea to remove talk about them in the Spree Guides. Also, while I was there, Spree 2.3.0 allows you to have a preferences column, added that to the Guides. And, as of a few hours ago, there's two new data types Spree now supports natively as arguments: `:array` and `:hash`.