docs: clarify TiCDC index DDL ordering

[doc-claw-bot]

First-time contributors' checklist

• I've signed the Contributor License Agreement, which is required for the repository owners to accept my contribution.

What is changed, added or deleted? (Required)

• Clarify in the TiCDC FAQ that the general DML/DDL execution order has an exception for ADD INDEX and CREATE INDEX when the downstream is TiDB.
• Update the TiCDC DDL page to explicitly call out these two DDLs as exceptions so the FAQ and DDL pages stay consistent.

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

• 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: docs: clarify TiCDC index DDL ordering docs#23336
• Other reference link(s):
  • https://docs.pingcap.com/zh/tidb/dev/ticdc-faq/
  • https://docs.pingcap.com/zh/tidb/dev/ticdc-ddl/

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

• 文档
  • 更新 TiCDC DDL 异步执行说明：在章节中更突出“TiDB 下 ADD INDEX / CREATE INDEX 属于例外”，说明 TiCDC 会在下游同步后立即返回，不等待 DDL 完成以避免阻塞后续 DML。
  • 调整 DML 与 DDL 执行顺序说明：在 如何理解 DML 和 DDL 之间的执行顺序？ 中补充 TiDB 场景下 ADD INDEX / CREATE INDEX 的异步例外，并补充相关参考链接。

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📝 Walkthrough

Walkthrough

TiCDC 文档更新了 TiDB 下游 ADD INDEX 和 CREATE INDEX 的异步执行规则，并补充其作为默认 DML、DDL 执行顺序例外的说明及参考链接。

Changes

TiCDC 索引 DDL 执行顺序说明

Layer / File(s)	Summary
索引 DDL 异步规则说明 ticdc/ticdc-ddl.md, ticdc/ticdc-faq.md	更新 TiDB 下游 ADD INDEX、CREATE INDEX 的异步执行说明，并补充其作为 DML、DDL 默认顺序例外的参考链接。

Estimated code review effort: 1 (Trivial) | ~2 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	标题准确概括了本次对 TiCDC 索引 DDL 顺序说明的文档澄清。
Description check	✅ Passed	描述包含模板要求的变更说明、适用版本、相关链接和 AI 参与信息，内容基本完整。
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.

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Actionable comments posted: 1

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 79e082d and ac81ac2.

📒 Files selected for processing (1)

• ticdc/ticdc-ddl.md