v8.5.7: document CPU-aware hot region scheduling (#21758)

[ti-chi-bot]

This is an automated cherry-pick of #21758

This PR is translated from: pingcap/docs#23140

What is changed, added or deleted? (Required)

Document CPU-aware Hot Region scheduling for read hotspots introduced in v8.5.7:

• Add flow_cpu and cpu-read-rate output descriptions for pd-ctl hot.
• Add min-hot-cpu-rate, cpu-rate-rank-step-ratio, and CPU-aware read-priorities behavior for balance-hot-region-scheduler.
• Add troubleshooting guidance for CPU-aware read hotspot scheduling.
• Add the Store read cpu panel description in the PD Grafana dashboard doc.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)

Need cherry-pick to release-8.5 because the feature is requested for v8.5.7.

What is the related PR or file link(s)?

• Release note PR: [WIP] add v8.5.7 release notes docs#23061
• PD issue: scheduler: introduce read cpu dimension tikv/pd#5718
• TiKV issue: pd: report hot read cpu in heartbeat tikv/tikv#19373
• PD implementation: pd: report hot read cpu in heartbeat tikv/pd#10178, [cherry-pick] pd: report hot read cpu in heartbeat tikv/pd#10707
• TiKV implementation: pd: report hot read cpu in heartbeat tikv/tikv#19305, resource_metering, raftstore: split region cpu by scheduler and unified read pools tikv/tikv#19473, raftstore, resource_metering: report hot read CPU in heartbeat tikv/tikv#19625

Do your changes match any of the following descriptions?

• Add a release note
• Add or modify documentation for a feature

Which TiDB components does this PR affect?

• TiDB
• PD
• TiKV

How do you ensure that your changes are correct? (Required)

• Checked local /home/lhy1024/pd and /home/lhy1024/tikv implementation for fields, defaults, and version fallback behavior.
• Ran ./scripts/markdownlint pd-control.md troubleshoot-hot-spot-issues.md grafana-pd-dashboard.md.
• Ran git diff --check.

[qiancai]

/approve

[ti-chi-bot]

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