[INLONG-395] Create the 1.2.0 version document

[bluewang]

Fixes #395

where XYZ should be replaced by the actual issue number.

Motivation

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Modifications

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Verifying this change

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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

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