docs: update Transform examples

[SaxtonDrey]

While learning how to use Blueprinter, I encountered some confusion with the current code examples.
The concepts are well-explained, but there appears to be a discrepancy with the method names used in the examples.

Checklist:

• I have updated the necessary documentation
• I have signed off all my commits as required by DCO
• My build is green

[lessthanjacob]

Thanks for opening this!

Reading through these docs again, I think this example is unnecessarily confusing generally. We could remove the custom_columns method entirely, and rework dynamic_fields to make this a bit easier to follow (i.e. more explicit). Something like:

class User
  def dynamic_fields
    case role
    when :admin
      { 
        employer: employer,
        time_in_role: determine_time_in role
      }
    when :maintainer
      { 
        label: label,
        settings: generate_settings_hash
      }
    when :read_only
      { 
        last_login_at: last_login_at
      }
    end
  end
end

[SaxtonDrey]

Thanks for your review and suggestion!
I agree that the current example is too complicated, so I updated PR(I just only took in your suggestion 😄 ).
Let me know if there are any further adjustments needed.