Add sdg help page

In this page we will render a list of clickable Goals icons that will show their
targets and related local targets

Co-authored-by: Senen <senenrodero@gmail.com>

config/routes/sdg.rb

@@ -1,3 +1,4 @@
 namespace :sdg do
   resources :goals, param: :code, only: [:index, :show]
+  get :help, controller: "goals"
 end