docs: add KVService Grafana dashboard

[SSmallMonster]

Summary

• add a provisionable Grafana dashboard for KVService performance metrics
• cover request, storage I/O, RDMA, metadata, cache, and error signals
• add node_exporter panels for NVMe await, utilization, and filesystem capacity
• add redis_exporter panels for health, clients, and memory

Metric sources

• KVService Prometheus exporter
• node_exporter for host storage panels
• redis_exporter for Redis health panels

Validation

• parsed with jq
• checked panel titles, data sources, targets, variables, and grid positions
• git diff --cached --check