add v7.5.8 release notes (#21930)

[ti-chi-bot]

This is an automated cherry-pick of #21930

What is changed, added or deleted? (Required)

• Add releases/release-7.5.8.md for the TiDB 7.5.8 patch release notes.
• Add the TiDB 7.5.8 entry to TOC-tidb-releases.md, releases/_index.md, and releases/release-timeline.md.
• Fill in the TiDB 7.5.8 release date.
• This release-notes update is prepared on release-8.5 because the release-notes directory is maintained there and will be cherry-picked to master.

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

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 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: add v7.5.8 release notes docs#23757
• Other reference link(s): generated from doc_4afdcdb19e49_release-7.5.8-updated-by-ai.md

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

Summary by CodeRabbit

• 新功能
  • 新增 TiDB 7.5.8 版本发布说明，并更新版本目录与发布时间线。
  • 发布说明包含 PD 监控指标改进，以及 TiDB、TiKV、PD、TiFlash、BR 和 TiCDC 的多项问题修复。
• 文档
  • 更新 TiDB 7.5 版本历史和发布列表，补充 7.5.8 的发布日期及链接。
  • 移除 TiDB 7.5.7 发布说明中的“下载离线包”链接。

[ti-chi-bot]

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[coderabbitai]

📝 Walkthrough

Walkthrough

Changes

TiDB 7.5.8 发布

Layer / File(s)	Summary
新增 7.5.8 发布说明 releases/release-7.5.8.md	新增 2026-09-17 发布的 TiDB 7.5.8 说明，包含 PD 监控指标改进，以及 TiDB、TiKV、PD、TiFlash、BR 和 TiCDC 的错误修复。
更新版本索引与导航 TOC-pingkai.md, TOC-tidb-releases.md, releases/_index.md, releases/release-timeline.md, releases/release-7.5.7.md	新增 7.5.8 的目录、版本列表和时间线记录，并从 7.5.7 试用链接中移除“下载离线包”。

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Other

Suggested reviewers: qiancai

Merge Risk: 🔵 Low · up to 0475c

The public release timeline may present TiDB 7.5.8 in the wrong order; correcting the entry placement is a small documentation fix.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	标题“add v7.5.8 release notes (#21930)”准确概括了新增 TiDB 7.5.8 发布说明的主要变更，内容简洁明确。
Description check	✅ Passed	描述包含变更内容、适用版本、关联链接、AI 参与情况和变更类型勾选信息，整体符合模板要求。虽然未包含首次贡献者检查清单，且列出了模板中未出现的旧版本选项，但不影响本次描述的完整性。
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Advanced

Run ID: 68234fae-bc03-41c0-88d4-bb3cafb81452

📥 Commits

Reviewing files that changed from the base of the PR and between 924e58f and 0475c17.

📒 Files selected for processing (6)

• TOC-pingkai.md
• TOC-tidb-releases.md
• releases/_index.md
• releases/release-7.5.7.md
• releases/release-7.5.8.md
• releases/release-timeline.md

Included review availability: Your plan provides up to 4 included reviews per hour; 3 remain after this review.

[coderabbitai]

🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win

将 7.5.8 移到时间线顶部。

releases/release-timeline.md 按发布日期从新到旧排列。新增版本日期为 2026-09-17，但当前行位于 8.5.4（2025-11-27）之后、7.5.7（2025-09-04）之前。已有 8.5.7（2026-07-09）等版本也应排在它之后。请将该行移动到现有最新版本条目之前。(docs.pingcap.com)

Source: MCP tools