Add documentation on permissions testing

What does this MR do and why?

This change adds documentation to describe testing the addition of new custom role permissions so that Developers have guidance that they can use for deciding what tests are necessary.

#403154 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

1. gdk config set gitlab_docs.enabled true && gdk reconfigure && gdk restart gitlab_docs
2. open http://gdk.test:3005/ee/development/permissions/custom_roles.html#testing