[FLINK-16077][docs] Translate "Custom State Serialization" page into Chinese

[liboshuai01]

What is the purpose of the change

Translate Custom State Serialization page into Chinese

Verifying this change

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

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

[liboshuai01]

Hi, @leonardBang @fredia , Excuse me, do you have time to take a look at this?

[flinkbot]

CI report:

• 4723296 Azure: FAILURE

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