docs: add tidb_dml_max_execution_time system variable

[gengliqi]

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Add tidb_dml_max_execution_time system variable.

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Summary by CodeRabbit

• 新功能

  • 新增系统变量 tidb_dml_max_execution_time，用于限制事务型 INSERT、REPLACE、UPDATE、DELETE 和 COMMIT 的最长执行时间。
  • 支持会话级、全局级配置，并可通过 SET_VAR 调整。

• 文档

  • 补充计时范围、适用场景、检测精度及相关限制说明。
  • 在系统变量索引和超时指南中新增该变量的参考信息，并标注适用版本。

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📝 Walkthrough

Walkthrough

新增 tidb_dml_max_execution_time 文档，说明其配置属性、适用语句、计时范围、排除场景和超时行为，并补充索引与开发指南链接。

Changes

DML 执行时间限制文档

Layer / File(s)	Summary
系统变量定义与适用范围 system-variables.md	新增 tidb_dml_max_execution_time 的配置属性、适用的事务型 DML 和 COMMIT、计时范围、排除场景及超时行为说明。
文档索引与开发指南 system-variable-reference.md, develop/dev-guide-timeouts-in-tidb.md	新增系统变量索引条目，并在开发指南中说明变量的默认值、单位、适用范围和限制。

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Other

Suggested reviewers: qiancai

Merge Risk: ⚪ Minimal · up to 101a8

This documentation-only change is internally consistent and has no established 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_dml_max_execution_time 系统变量文档，与主要变更一致，简洁且具体。
Description check	✅ Passed	描述已填写必需的变更内容和适用版本，并完成贡献者协议及 AI 参与情况勾选。可选的关联链接为空不影响完整性。
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.

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