administrate_collapsible_navigation

Add this line to your application's Gemfile:

gem 'administrate_collapsible_navigation'

And then execute:

$ bundle install

By default this gem modifies the default app/views/admin/application/index.html.erb (show.html.erb, edit.html.erb, and new.html.erb) and inserts the toggle icon here.

If you have this view already overwritten, you can also just drop <%= image_tag 'menu_open.png', id: 'navigation__toggle' %> in the overwritten view in the same place in the code, next to the h1 resource title.