Add Analytics Dashboard landing page to Explore navigation

Background

Add the new Analytics Dashboard landing page as a top-level navigation item under the Explore menu. This provides users with a single, discoverable entry point to access all dashboards without navigating through multiple menus.

TODO

• Create new menu item for Analytics dashboards
• Add a new feature flag explore_analytics_dashboards
• Create feature flag rollout issue #592866
• Add some basic RSpec tests