Add analytics dashboards explore route

Summary of Changes

• This MR adds a new Analytics Dashboards menu item to the Explore navigation. It creates a landing page with an empty state that serves as the entry point for accessing analytics dashboards.
• The feature is controlled by a feature flag (explore_analytics_dashboards)

Code Details

The implementation includes:

Controller (app/controllers/explore/analytics_dashboards_controller.rb):

• Simple controller with an index action that renders the landing page
• Feature flag check that returns 404 if the flag is disabled

View (app/views/explore/analytics_dashboards/index.html.haml):

• Empty state component with an illustration and title
• Placeholder for future dashboard content

Sidebar Menu (lib/sidebars/explore/menus/analytics_dashboards_menu.rb):

• Menu item added to the Explore sidebar
• Uses a chart icon and respects the feature flag for visibility
• Integrated into the Explore panel navigation

Feature Flag (config/feature_flags/wip/explore_analytics_dashboards.yml):

• WIP flag set to disabled by default
• Linked to issue #589181 (closed)

Routes (config/routes/explore.rb):

• Added resource route for analytics_dashboards with index action only

Tests (spec/requests/explore/analytics_dashboards_controller_spec.rb):

• Tests verify the controller responds with 200 when the feature flag is enabled
• Tests verify 404 response when the feature flag is disabled

Screenshots or screen recordings

How to set up and validate locally

1. Enable the feature flag in your local environment:

   Feature.enable(:explore_analytics_dashboards)

2. Navigate to the Explore menu in the GitLab UI and verify the "Analytics dashboards" menu item appears

3. Click the menu item and confirm it loads the empty state page at /explore/analytics_dashboards

4. Disable the feature flag and verify the menu item disappears and the route returns a 404

5. Run the RSpec tests:

   bundle exec rspec spec/requests/explore/analytics_dashboards_controller_spec.rb

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #589181 (closed)