Add Rubocop

[the-teacher]

In this PR I add rubocop to provide quality of the code.

• I found that adding # frozen_string_literal: true is still good practice in Ruby3/Rails7 Link
  I do not actually like that # frozen_string_literal: true affected default rails files. But no problem.
• I found new syntax content_raw:
• I had to review files to decide what files to exclude from checks. I wanted to avoid some checks of default rails files, also decided to avoid some checks for file that I've created for the project.