doc: Document move of TiCDC to separate repo

[dveeden]

What problem does this PR solve?

Issue Number: close #12121

What is changed and how it works?

Check List

Tests

• No code

Questions

Will it cause performance regression or break compatibility?

Do you need to update user documentation, design documentation or monitoring documentation?

Release note

None

[dveeden]

/cc @lance6716

[lance6716]

/cc @flowbehappy

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[LGTM Timeline notifier]

Timeline:

• 2025-11-12 10:14:46.861972891 +0000 UTC m=+869936.305002770: :ballot_box_with_check: agreed by lidezhu.
• 2025-11-12 10:14:53.66919665 +0000 UTC m=+869943.112226529: :ballot_box_with_check: agreed by hongyunyan.

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: 3AceShowHand, flowbehappy, hongyunyan, lidezhu, wk989898

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