add ddl embedded analyze related document.

[AilinKid]

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[Copilot]

Pull Request Overview

This PR introduces documentation for the DDL Embedded Analyze optimization feature in TiDB v8.5.4. The feature allows DDL operations involving index creation or reorganization to automatically trigger inline statistics analysis.

Key Changes:

• Adds a new system variable tidb_stats_update_during_ddl to control DDL embedded analyze behavior
• Creates comprehensive documentation explaining DDL Embedded Analyze optimization for ADD INDEX and column modification operations
• Includes practical SQL examples demonstrating the feature for both new index creation and existing index reorganization scenarios

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
system-variables.md	Adds documentation for the new tidb_stats_update_during_ddl system variable introduced in v8.5.4
ddl_embedded_analyze.md	New documentation file explaining DDL Embedded Analyze optimization with detailed examples for index creation and column modification operations

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[AilinKid]

@hfxsd can this be merged before Fri? one of the big customer wait for this document in v8.5.4

[tangenta]

应该重点介绍需要开启这个变量的使用场景；演示的话 create index 和 modify column 选一个展示效果比较好。

[AilinKid]

在开头加了一个场景背景章节

[tangenta]

注释去掉，用中文写在正文里？

[AilinKid]

好的

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-10-31 06:56:35.821661885 +0000 UTC m=+1632501.898914465: ☑️ agreed by tangenta.