[FLINK-27909][doc] Add document for hybrid shuffle

[reswqa]

What is the purpose of the change

Add document for hybrid shuffle

Brief change log

• Add hybrid shuffle to taskmanager.memory.framework.off-heap.batch-shuffle.size description.
• Add document for hybrid shuffle mode.

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): yes
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? docs

[flinkbot]

CI report:

• 999c4d0 Azure: SUCCESS

Bot commands

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[xintongsong]

Thanks @reswqa. LGTM.

[xintongsong]

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[xintongsong]

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[xintongsong]

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[reswqa]

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