add the release notes for v8.5.8

[qiancai]

What is changed, added or deleted? (Required)

As in the title

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: add the release notes for v8.5.8 docs#23522
• Other reference link(s):

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

  • 发布 TiDB 8.5.8，涵盖 TiDB、TiKV、PD、BR、TiCDC 和 TiDB Lightning 的功能更新。
  • 优化变更数据捕获扫描与批处理，并提升 Event Service 稳定性。

• 性能改进

  • 改进查询优化器估算、内存管理及相关系统性能。

• 问题修复

  • 修复 DDL、查询执行、事务、索引一致性、Raft、资源组及安全相关问题。
  • 改善备份恢复、故障转移、下游数据一致性和 IMPORT INTO 的稳定性。

[coderabbitai]

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📝 Walkthrough

Walkthrough

新增 TiDB 8.5.8 发布说明，涵盖 TiDB、TiKV、PD、BR、TiCDC 和 TiDB Lightning 的性能改进及错误修复。

Changes

TiDB 8.5.8 发布说明

Layer / File(s)	Summary
版本结构与性能改进 releases/release-8.5.8.md	新增版本信息、发布说明章节结构，以及 TiDB 和 TiCDC 的性能改进条目。
核心组件错误修复 releases/release-8.5.8.md	新增 TiDB、TiKV 和 PD 的错误修复列表。
备份、Changefeed 与导入修复 releases/release-8.5.8.md	新增 BR、TiCDC 和 TiDB Lightning 的错误修复列表。

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

Merge Risk: 🟡 Moderate · up to 47c9a

The release notes still contain a placeholder date, lack a compatibility alias, include entries that upstream marked as not requiring release notes, and use unclear wording for a unique-constraint consequence. The page is therefore not ready to publish until these documentation issues are corrected.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	描述包含模板的大部分章节，但必填的变更说明仅写为“As in the title”，内容不足；版本选择部分未勾选任何版本，也未明确说明目标为 v8.5 分支。	补充具体的变更内容和变更原因。勾选 v8.5，并确认是否需要勾选 master。

✅ Passed checks (4 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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.
Title check	✅ Passed	标题准确概括了主要变更，即新增 TiDB v8.5.8 发布说明。标题简洁且与改动内容一致。

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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Actionable comments posted: 3

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Actionable comments posted: 2

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[ti-chi-bot]

@YangKeao: adding LGTM is restricted to approvers and reviewers in OWNERS files.

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[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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Needs approval from an approver in each of these files:

• OWNERS [qiancai]

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch master: #21897.
But this PR has conflicts, please resolve them!