Add an Explore dashboards banner to the legacy dashboards list

What does this MR do and why?

• Adds a dismissible tip alert above the heading on the legacy group and project Analytics dashboards pages.
• Points people at the instance-wide Explore dashboards space (/explore/analytics_dashboards) with a "Go to Explore" button and a "Learn more" link to the docs.
• New explore_dashboards_alert.vue component plus a Storybook story, same shape as the old DORA/MR analytics migration alerts.
• Backend only sends the Explore path when explore_analytics_dashboards is on for the user — same flag as the Explore controller, so the link can't dangle. No flag check needed in the Vue component.
• Dismissal is its own callout, explore_analytics_dashboards_promo.

The UX mockup talks about dashboards "moving to a central location" and wants a warning that the legacy page is going away. Skipped that — deprecating the legacy pages still needs Breaking-Change-Exception approval, and cross-group dashboards in one view isn't shipped yet either, so the copy doesn't promise either.

References

UX spec: https://gitlab.com/gitlab-org/gitlab/-/work_items/527178 Legacy dashboards deprecation (Breaking-Change-Exception): https://gitlab.com/gitlab-org/gitlab/-/work_items/626531

Screenshots or screen recordings

Before	After	Banner on its own

Rendered from Storybook with mocked dashboards, so there's no sidebar or breadcrumb.

How to set up and validate locally

1. Feature.enable(:explore_analytics_dashboards) in a Rails console.
2. Visit a group's Analytics dashboards page at /groups/<group>/-/analytics/dashboards, and the project equivalent.
3. Confirm the alert shows above the heading, "Go to Explore" opens /explore/analytics_dashboards, and dismissing it survives a reload.
4. Feature.disable(:explore_analytics_dashboards) and confirm the alert's gone.

MR acceptance checklist

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