Add hint about the moved menu entry

What does this MR do and why?

Describe in detail what your merge request does and why.

The Pages menu position experiment do far did not include a hint that the menu entry had moved, confusing some users. This MR introduces a callout for that.

Screenshots or screen recordings

How to set up and validate locally

1. Enable the feature

    Feature.enable(:show_pages_in_deployments_menu)

2. Visit any project's settings page such as http://127.0.0.1:3000/flightjs/Flight/edit
3. Observe the callout

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.