grafana-performance-overview-dashboard.md describes TiDB CPU / TiKV CPU/IO MBps panels that no longer exist

Error Report

1. What is the URL/path of the document related to this issue?

• https://docs.pingcap.com/tidb/dev/grafana-performance-overview-dashboard (grafana-performance-overview-dashboard.md)
• Also present identically on the release-8.5 branch.

2. How would you like to improve it?

The "Performance Overview" section of this page has separate ### TiDB CPU and ### TiKV CPU/IO MBps panel descriptions. These panels no longer exist under these names in the actual Grafana dashboard.

Verified two ways:

• The real dashboard JSON (performance_overview.json in pingcap/monitoring) only has TiDB CPU/Memory, TiKV CPU/Memory, and PD CPU/Memory panels — there is no standalone TiDB CPU or TiKV CPU/IO MBps panel.
• Confirmed live by starting tiup playground --monitor and opening the actual Grafana "Performance Overview" dashboard: it shows combined TiDB CPU/Memory, TiKV CPU/Memory, and PD CPU/Memory panels (CPU and Memory on one panel), not the separate CPU-only / CPU+IO panels the doc describes.

This dashboard is originally authored in pingcap/tidb at pkg/metrics/grafana/performance_overview.json, and the rename was made in pingcap/tidb#54978 ("metrics: add memory info and traffic flow into performance overview", merged 2024-07-30). That PR's diff shows:

-      "title": "TiDB CPU",
+      "title": "TiDB CPU/Memory",
...
-      "title": "TiKV CPU/IO MBps",
+      "title": "TiKV CPU/Memory",
...
+      "title": "PD CPU/Memory",   (newly added panel)

So the doc has been describing panels that stopped existing under those names since this PR merged in mid-2024 (over a year ago).

Could someone update these two sections (and the corresponding PD CPU/Memory panel, which isn't documented as a separate section at all) to match the current panel structure? Happy to help verify the new panel's actual sub-metrics if useful.

(Found while reviewing the Japanese translation of this page.)

Update grafana-performance-overview-dashboard.md, focusing on the Performance Overview section. Compare its panel descriptions with performance_overview.json in pingcap/monitoring and, if needed, verify using tiup playground --monitor. Done means the documentation describes TiDB CPU/Memory, TiKV CPU/Memory, and the previously missing PD CPU/Memory panel.