[INLONG-295] Add document for introducing how to extend a new type of sink in sort

[ApocalypseWan]

Fixes #295

where XYZ should be replaced by the actual issue number.

Motivation

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Modifications

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Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

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This change is already covered by existing tests, such as (please describe tests).

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This change added tests and can be verified as follows:

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• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)
• If a feature is not applicable for documentation, explain why?
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation

[dockerzhang]

@chantccc lack of Chinese Document.

[ifndef-SleePy]

Nice work