Add Valkey support to Winter CMS documentation

Summary

• Add comprehensive documentation for Valkey as a cache and session driver
• Include installation, configuration, and usage examples
• Update existing cache documentation to include Valkey alongside Redis options
• Document Valkey as a Redis-compatible alternative for caching, queues, and sessions

Files to Modify/Add

New Documentation Files:

• services/cache.md - Update cache service documentation to include Valkey configuration options
• services/queues.md - Update queues service documentation with Valkey support details
• services/session.md - Update session service documentation with Valkey configuration
• setup/configuration.md - Add Valkey environment variables