Align stretch clusters docs with multicluster operator lab

Major updates based on the official multicluster operator lab guide:

StretchCluster spec:
- Change spec.clusterSpec.network.mode to spec.networking.crossClusterMode
- Add tls, auth.sasl, storage, resources, enterprise, listeners sections
- Add cluster config tuning for WAN latency (node_status_interval,
  raft_heartbeat_timeout_ms, replicate_append_timeout_ms, etc.)

NodePool spec:
- Change spec.clusterName to spec.clusterRef with group/kind/name
- Add image section with repository and tag
- Add services.perPod for Cilium global service annotations
- Add podTemplate.spec.nodeSelector for topology awareness

Bootstrap command:
- Add --loadbalancer flag for automatic LoadBalancer Service provisioning
- Add --service-name and --create-namespace flags
- Add flags reference table
- Remove manual kubeconfig merge step (bootstrap handles this)

Authentication:
- Add SASL credential Secret creation step before StretchCluster
- Use SCRAM-SHA-512 mechanism

Other improvements:
- Consolidate to single 'redpanda' namespace for operator and workloads
- Update license secret key to 'license.key' for consistency
- Fix troubleshooting section to reference clusterRef instead of clusterName
- Note that CRDs are installed via crds.experimental=true

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: JakeSCahill