Do not merge until v8.5.4: reference: update doc for follower read (#21962)

[ti-chi-bot]

This is an automated cherry-pick of #21962

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• This PR is translated from: reference: update doc for follower read docs-cn#20703
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[gemini-code-assist]

Summary of Changes

Hello @ti-chi-bot, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the documentation for TiDB's Follower Read feature. It provides clearer explanations of its purpose, implementation mechanisms, and configuration options, including new platform-specific details for replica selection and updated guidance on the tidb_replica_read system variable. The updates also introduce new monitoring metrics and discuss performance considerations, aiming to improve user understanding and effective utilization of Follower Read.

Highlights

• Enhanced Follower Read Documentation: The follower-read.md document has been significantly updated to provide more comprehensive details on the Follower Read feature, including its purpose, implementation, and configuration.
• Platform-Specific Replica Selection Details: New CustomContent blocks have been added to explain how TiDB selects appropriate replicas based on zone labels for both TiDB (self-managed) and TiDB Cloud environments, enhancing clarity for different deployment types.
• New Usage Scenarios and Goals: The document now includes dedicated sections outlining suitable usage scenarios for Follower Read and its achieved goals, such as distributing read hotspots and prioritizing local replica reads.
• Updated tidb_replica_read Variable Explanation: The tidb_replica_read system variable documentation has been revised to include recommended configurations (leader, closest-adaptive, closest-replicas) for reducing cross-AZ traffic, along with a compatibility note for versions v8.5.4 and v9.0.0.
• Detailed Replica Selection Strategies: The 'Implementation mechanism' section now elaborates on specific follower replica selection strategies: leader, closest-replicas, and closest-adaptive, including their behaviors and implications.
• Follower Read Performance Overhead: A new section has been added to discuss the performance overhead of Follower Read, particularly for small queries, and provides recommendations for its optimal use.
• Monitoring Guidance: A 'Basic monitoring' section has been introduced, guiding users to the 'KV Request > Read Req Traffic' panel in Grafana for observing Follower Read effects, with platform-specific links.
• New Grafana Metrics: The grafana-tidb-dashboard.md file has been updated to include new 'Read Req Traffic' metrics, providing more granular visibility into read request distribution (leader/follower, local/cross-zone).
• tidb_replica_read Scope Clarification: The system-variables.md document now clarifies that the tidb_replica_read variable only affects read-only SQL statements starting from v8.5.4 and v9.0.0.

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[gemini-code-assist]

Code Review

This pull request updates the documentation for the Follower Read feature. I've reviewed the changes and made a few suggestions to improve grammar, clarity, and consistency, following the repository's style guide. The changes mostly involve minor wording improvements for better readability and a correction in the grafana-tidb-dashboard.md file to ensure metric descriptions are consistent.

[ti-chi-bot]

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