GitLab Migration: Add visibility settings warning

What does this MR do and why?

This adds a warning to GitLab Migration about the visibility settings/rules and updates our handbook.

Related to #331252 (closed)

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #331252 (closed)