[WIP]: add v7.5.8 release notes

[doc-claw-bot]

What is changed, added or deleted? (Required)

• Add releases/release-7.5.8.md for the TiDB 7.5.8 patch release notes.
• 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)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)

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

• This PR is translated from: [WIP]: add v7.5.8 release notes docs#23757
• Other reference link(s): generated from doc_4afdcdb19e49_release-7.5.8-updated-by-ai.md

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 发布说明，包含版本信息、试用与部署链接。
  • PD 集群状态监控指标新增 Store 维度标签，提供更细致的监控信息。

• 问题修复

  • 修复 UNION 查询崩溃、规划阶段 CPU 占用过高及统计信息加载异常。
  • 修复 TiKV、PD、TiFlash、TiCDC 和 Lightning 导入相关的崩溃、内存耗尽、吞吐下降、数据不一致及资源泄漏问题。

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• releases/release-7.5.8.md

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📝 Walkthrough

Walkthrough

新增 TiDB 7.5.8 发布说明文档。文档包含版本信息、部署链接、PD 监控指标改进，以及 TiDB、TiKV、PD、TiFlash 和 TiCDC 的错误修复条目。

Changes

TiDB 7.5.8 发布说明

Layer / File(s)	Summary
发布说明基础信息 releases/release-7.5.8.md	新增 front matter、版本信息、发布日期占位，以及试用、生产部署和离线包下载链接。
监控改进与 TiDB 修复 releases/release-7.5.8.md	记录 pd_cluster_status 新增 store 标签，以及 TiDB 中 UNION 查询崩溃、规划期 CPU 消耗过高和异步加载统计信息 panic 的修复。
组件错误修复 releases/release-7.5.8.md	记录 TiKV、PD、TiFlash 和 TiCDC 的错误修复，包括 panic、GC safepoint、内存配额、数据一致性和 Kafka client 泄漏问题。

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~3 minutes

Change: Other

Merge Risk: ⚪ Minimal · up to 99ba8

This change adds release notes only, with no identified merge-blocking risk.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	标题明确说明新增 TiDB v7.5.8 发布说明，与本次变更的主要内容一致。
Description check	✅ Passed	描述包含变更内容、适用版本和相关链接，整体符合模板要求。描述未包含模板中的“AI agent involvement”部分，且版本勾选与 TiDB 7.5.8 发布说明的关系需要进一步确认，但不影响描述的基本完整性。
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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Actionable comments posted: 1

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• releases/release-7.5.8.md

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