add active storage has many guide

docs/2.0/recipes/use-the-has-many-field-for-active-storage-attachments.md

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+# Use the `has_many` field for Active Storage attachments
+
+Because [Active Storage](https://edgeguides.rubyonrails.org/active_storage_overview.html) is implemented as an association.
+
+For that you should create an `attachment_resource.rb` and configure it like so.
+
+```ruby
+class AttachmentResource < Avo::BaseResource
+  self.title = :filename
+  self.model_class = "ActiveStorage::Attachment"
+
+  field :id, as: :id
+  field :filename, as: :text
+  field :service_url, as: :external_image, name: "Image"
+  field :created_at, as: :date_time
+end
+```
+
+Next, add it to your resource that `has_many_attached`:
+
+```ruby
+class Project < ApplicationRecord
+  has_many_attached :files
+end
+
+class ProjectResource < Avo::BaseResource
+  # Please make sure you use the _attachments suffix.
+  # So the :files key from the model file becomes :files_attachments on the resource file.
+  field :files_attachments, as: :has_many
+end
+```
+
+![](/assets/img/recipes/use-the-has-many-field-for-active-storage-attachments/preview.png)
+
+We suggest making this resource read-only using Pundit policies so you don't accidentaly update Active Storage keys and settings.