topsql: document detailed TiKV I/O dimensions

[jiong-nba]

What is changed, added or deleted? (Required)

• Document the new detailed TiKV I/O dimensions in Top SQL, including the switch dependency, UI behavior, compatibility fallback, overhead, and troubleshooting.
• Document resource-metering.enable-detailed-io-collection in the TiKV configuration reference.
• Clarify that Read IOPS is a foreground RocksDB block-read attribution signal rather than device-level IOPS.
• Remove two outdated Top SQL screenshots that no longer match the settings and sorting controls.
• The target release has not been confirmed yet, so this PR targets master only and does not add a "New in vX.Y.Z" marker.

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: topsql: document detailed TiKV I/O dimensions docs-cn#21886
• 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
  • ng-monitoring: topsql: support detailed IO dimensions ng-monitoring#355

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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