docs: Redis Cluster announce endpoint guide for external access

[ArnobKumarSaha]

Summary

• Document the Announce Redis Endpoints toggle for Cluster-mode Redis in the UI, so users understand why/when they need it and what to fill in (Type, per-shard endpoints).
• Explains the underlying problem: the default external Gateway endpoint load-balances across all shards, breaking Redis Cluster slot redirects for external clients.
• Clarifies that all backend/installation setup is already handled; UI users only need to supply the externally reachable endpoints per shard.
• Links to the kubedb backend exposure guide for further detail.

Test plan

• Rendered markdown reviewed for correct links/anchors
• Docs site build (if run in CI)