RS & k8s: Enabling clustering for Active-Active DBs after creation

[rrelledge]

Note

Low Risk
Documentation-only changes with no runtime, API, or security impact.

Overview
Updates Redis Software and Kubernetes docs so Active-Active databases can enable clustering/sharding after creation, while still stating clustering cannot be turned off later.

For RS, creation and manage pages document crdb-cli crdb update with shards_count in default-db-config, plus warnings not to change shard count by other means (sync risk). For K8s REAADB, the note now says to raise shardCount in the REAADB spec instead of implying sharding is permanently disabled when created with one shard.

^{Reviewed by Cursor Bugbot for commit 93791a0. Bugbot is set up for automated code reviews on this repo. Configure here.}

[github-actions]

DOC-6666

[github-actions]

Staging links:
https://redis.io/docs/staging/DOC-6666/operate/kubernetes/active-active/create-reaadb/
https://redis.io/docs/staging/DOC-6666/operate/rs/databases/active-active/create/
https://redis.io/docs/staging/DOC-6666/operate/rs/databases/active-active/get-started/
https://redis.io/docs/staging/DOC-6666/operate/rs/databases/active-active/manage/

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

---

^{Security scan by Jit}

[dwdougherty]

LGTM.