docs: clarify TiCDC index DDL ordering (#21829)

[ti-chi-bot]

This is an automated cherry-pick of #21829

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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