ticdc: add more docs about unified sorter

[overvenus]

First-time contributors' checklist

• I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

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

• master (the latest development version)
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[overvenus]

/cc @liuzix @amyangfei @TomShawn

[TomShawn]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn
• amyangfei

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[TomShawn]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 095771b

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #6365.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #6366.

[Joyinqin]

/translation done
/remove-translation doing

[ti-chi-bot]

In response to a cherrypick label: new pull request could not be created: failed to create pull request against pingcap/docs-cn#release-4.0 from head ti-chi-bot:cherry-pick-6258-to-release-4.0: status code 422 not one of [201], body: {"message":"Validation Failed","errors":[{"resource":"PullRequest","code":"custom","message":"A pull request already exists for ti-chi-bot:cherry-pick-6258-to-release-4.0."}],"documentation_url":"https://docs.github.com/rest/reference/pulls#create-a-pull-request"}

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #6369.

[ti-chi-bot]

In response to a cherrypick label: new pull request could not be created: failed to create pull request against pingcap/docs-cn#release-4.0 from head ti-chi-bot:cherry-pick-6258-to-release-4.0: status code 422 not one of [201], body: {"message":"Validation Failed","errors":[{"resource":"PullRequest","code":"custom","message":"A pull request already exists for ti-chi-bot:cherry-pick-6258-to-release-4.0."}],"documentation_url":"https://docs.github.com/rest/reference/pulls#create-a-pull-request"}

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #6370.