Add detail example for adding 'User Session Count Limit' execution to the browser authentication flow

[HwangTaehyun]

Hi, Thank you for the amazing open source.

Recently, I added user session limit execution flow with my keycloak setting with the following reference.

• https://www.keycloak.org/docs/latest/server_admin/index.html#_step-up-flow:~:text=throw%20an%20error.-,User%20session%20limits,-Limits%20on%20the
• https://github.com/keycloak/keycloak/issues/10077

However, It isn't very clear for me to add this feature to the authorization flow. It would be more helpful if there are more examples or additional explanations.

Thank you! @mposolda @andymunro

Please review this PR @mposolda @andymunro