cloud: document detailed Top SQL I/O dimensions

[jiong-nba]

What is changed, added or deleted? (Required)

• Document the detailed Top SQL I/O dimensions available in TiDB Cloud Clinic for TiDB Cloud Dedicated clusters hosted on AWS and running TiDB v9.0.0 or later.
• Explain that Logical Read, Logical Write, and Read IOPS replace the combined Logical IO sort dimension, while CPU and Network remain available.
• Clarify that Read IOPS is a foreground RocksDB block-read attribution signal rather than device-level IOPS.
• Update the Top SQL usage step to include the detailed I/O dimensions.

This PR targets release-8.5 because TiDB Cloud documentation is maintained on that branch for content reuse. The documented feature applies to eligible clusters running TiDB v9.0.0 or later.

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

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions.

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)

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

• This PR is translated from: N/A
• Other reference link(s):
  • Tracking issue: Extend Top SQL resource dimensions with logical read/write and RocksDB block reads tikv/tikv#19867
  • TiKV: *: extend Top SQL resource dimensions tikv/tikv#19953
  • kvproto: resource_usage_agent: add RocksDB block read count to Top SQL resource dimensions kvproto#1498
  • TiDB Dashboard: topsql: support detailed TiKV IO dimensions tidb-dashboard#1917
  • Clinic UI: https://github.com/pingcap-inc/clinic-ui/pull/536
  • ng-monitoring: topsql: support detailed IO dimensions ng-monitoring#355
  • TiDB documentation: topsql: document detailed TiKV I/O dimensions #23532

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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