Update image size customization docs

We found that the documentation on customizing image sizes for products and
variants was outdated and suggested users introduce a decorator. This was
fixed previously in the Legacy guides and we have ported the updated documentation
to the new version here.

Fixes #94

Co-authored-by: Cameron Day <[email protected]>

docs/advanced-solidus/images-and-image-processing.mdx

@@ -42,38 +42,48 @@ image = Spree::Image.first
 image.url(:product)
 ```
 
-If you're building a custom storefront, you may also want to change the sizes of the images in your
-store. You can do this by applying an override to `Spree::Image`:
+If you're building a custom storefront, you may also want to change the sizes of
+the images in your store or add additional sizes. The default sizes can be changed
+using the `Spree::Config.product_image_styles` option.
 
-```ruby title="app/overrides/amazing\_store/spree/image/customize\_sizes.rb"
-module AmazingStore
-  module Spree
-    module Image
-      module CustomizeSizes
-        def self.prepended(klass)
-          klass.attachment_definitions[:attachment][:styles] = {
-            mini: '48x48>',
-            small: '400x400>',
-            product: '680x680>',
-            large: '1200x1200>',
-            jumbo: '1600x1600>'
-          }
-        end
-
-        ::Spree::Image.prepend self
-      end
-    end
-  end
-end
+For example, in your `config/initializers/spree.rb` file. You can set
+new defaults like this:
+
+```ruby
+# config/initializers/spree.rb
+# E.g. these are the default values for Solidus up to version 3.3
+config.product_image_styles = {
+  mini: '48x48>',
+  small: '100x100>',
+  product: '240x240>',
+  large: '600x600>',
+  jumbo: '1600x1600>'
+}
 ```
 
+Similar to product and variant image styles, you can customize the taxon image
+styles using the `Spree::Config.taxon_image_styles` configuration option.
+
+:::caution
+
+[Active Storage][activestorage] will automatically generate the sizes upon
+initial request. If you change the default image sizes and are using Paperclip,
+you must regenerate the thumbnails by running a Rake task:
+
+```bash
+bundle exec rake paperclip:refresh:thumbnails CLASS=Spree::Image
+```
+
+:::
+
 Now that you changed your sizes, try getting the URL for your new `jumbo` size or `large` sizes:
 
 ```ruby
-image = Spree::Image.first
+image = Spree::Product.first.images.first
 image.url(:jumbo)
-taxon = Spree::Taxon.first
-taxon.url(:large)
+
+icon = Spree::Taxon.first.icon
+icon.url(:mini)
 ```
 
 Solidus will generate the new sizes and return their URLs!