docs: clarify TiCDC index DDL ordering (#23336)

[ti-chi-bot]

This is an automated cherry-pick of #23336

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:
• Other reference link(s):
  • docs: clarify TiCDC index DDL ordering docs-cn#21829
  • https://docs.pingcap.com/tidb/dev/ticdc-faq/
  • https://docs.pingcap.com/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

• Documentation
  • Clarified TiCDC’s execution order for DDL vs. DML replication as a three-step flow anchored to the DDL commit timestamp.
  • Documented that replication pauses for tables affected by DDL until the DDL commit completes.
  • Added TiDB-specific note that ADD INDEX and CREATE INDEX run asynchronously, allowing subsequent DML to proceed without waiting for completion.

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/approve

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