Chore: Add example data migration

Because:
- When I need to write a new data migration, I want to have an example migration I can reference and use to copy/paste the boilerplate.

lib/tasks/data_migrations/example.rake

@@ -0,0 +1,21 @@
+# create new data migration rake task file and copy the contents of this file into it
+#
+# namespace :data_migrations do
+#
+#   desc "<What will this task do?>"
+#   task example_data_migration_name: :environment do
+#     puts "Processing #{Lesson.count} lessons"
+#
+#     progressbar = ProgressBar.create(total: Lesson.count, format: '%t: |%w%i| Completed: %c %a %e')
+#
+#     Lesson.find_each do |lesson|
+#       begin
+#         # do something
+#       rescue StandardError => e
+#         progressbar.log "There was an error with lesson ##{lesson.id}: #{e.message}"
+#       ensure
+#         progressbar.increment
+#       end
+#     end
+#   end
+# end