Add documentation for Maven virtual registry UI

Problem

The Maven Virtual Registry now features a user interface that enables users to manage virtual registries and upstreams through the GitLab UI; however, the documentation currently only covers API-based management. Users need comprehensive documentation for the new UI functionality.

Current UI Capabilities

The new UI allows users to:

• CRUD virtual registries: Create, edit, and delete virtual registries
• CRUD upstream registries: Add, edit, and delete upstream registries
• Test upstream connections: Verify connectivity to upstream registries
• Reorder upstreams: Change the priority order of upstream registries
• Browse cached packages: View and search packages cached from upstreams
• View cache metadata: See creation time, size, and source for cache entries
• Delete cache entries: Remove specific items from the cache

Proposal

Add comprehensive UI documentation to the Maven Virtual Registry page (/user/packages/virtual_registry/maven/) that covers:

New "Manage virtual registries in the UI" section

• How to navigate to the virtual registries interface
• Creating a new virtual registry
• Editing virtual registry details
• Deleting a virtual registry

New "Manage upstream registries in the UI" section

• Adding new upstream registries
• Configuring upstream settings (name, URL, credentials, cache validity)
• Editing existing upstreams
• Testing upstream connections
• Reordering upstream priority
• Deleting upstreams

New "Browse and manage cached packages" section

• Viewing cached packages by upstream
• Searching cached packages
• Viewing cache metadata
• Deleting specific cache entries

Updated structure

• Keep existing API documentation alongside new UI sections
• Maintain all existing client configuration documentation
• Preserve Maven-specific requirements and restrictions

Benefits

1. Complete feature coverage - Documents all UI capabilities for virtual registry management
2. Improved user experience - Users can choose between UI and API approaches
3. Better discoverability - UI procedures are clearly documented with step-by-step instructions
4. Reduced support burden - Users have self-service documentation for common tasks