Bring back hovering - 1st iteration (behind feature flag)

What does this MR do and why?

This MR brings back the on-hover flyout menus as we have them in the old navigation 🦋 When hovering the mouse pointer over a section name, the section's items are shown in a floating menu next to it. This allows faster no-click scanning of all sections and one-click navigation again.

recording_1688728166

How to set up and validate locally

• Enable the feature flag in the gdk rails console: Feature.enable(:super_sidebar_flyout_menus)
• Browse to any project or group.
• While your browser window with is >= 768px, when you hover a section title like "Manage" or "Code" you should see a flyout menu open to the right. Hover to another section, see another flyout.
• You can still click the section title to open it. This closes the flyout and shows the same contents in the expanded section instead (as it was before this MR).
• Verify that flyouts are not appearing in windows smaller than 768px.
• Verify that flyouts are not appearing when with touch pointer events. Either open your GDK on a real phone/tablet if that works in your network, or use your browser dev tools to simulate a touch devise.

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 #415343 (closed)