add documents preview endpoint

config/routes.rb

@@ -34,6 +34,7 @@ Rails.application.routes.draw do
     resources :submitter_form_views, only: %i[create]
     resources :submitters, only: %i[index show update]
     resources :submissions, only: %i[index show create destroy] do
+      resources :documents, only: %i[index], controller: 'submission_documents'
       collection do
         resources :init, only: %i[create], controller: 'submissions'
         resources :emails, only: %i[create], controller: 'submissions', as: :submissions_emails