Add documentation to provide a guide for how to use pundit with rails 8's built-in authentication generator

[SuzukiRyuichiro]

Please disregard if this is not a necessary update on the documentation.
I was testing the combination of pundit and the new rails 8 authentication generator. If you use the built in authentication generator, you would get the currently authenticated user via Current.user not current_user. Since pundit expect current_user, some people suggesting to add that helper to the framework. rails/rails#54202

I just thought it might be a good thing to point out that it could be resolved with the portion that is already explained in the README.

My suggestion is just to make it more obvious by giving the exact solution.

To do

• I have read the contributing guidelines.
• I have added relevant tests.
• I have adjusted relevant documentation.
• I have made sure the individual commits are meaningful.
• I have added relevant lines to the CHANGELOG.

[Burgestrand]

Thank you, this looks great! I agree.