[FLINK-37658][docs] Improve the Datagen connector docs

[davidradl]

Amend the DataGen connector docs as agreed in PR 26437.

I have added the example from the PR into the docs and have made small improvements to the text. I have added a Chinese version of the doc change.

What is the purpose of the change

Include a length example for collections - which now works after PR 26437.
Also make minor improvements to the test in this section.

Brief change log

(for example:)

• The TaskInfo is stored in the blob store on job creation time as a persistent artifact
• Deployments RPC transmits only the blob storage reference
• TaskManagers retrieve the TaskInfo from the blob cache

Verifying this change

Please make sure both new and modified tests in this PR follow the conventions for tests defined in our code quality guide.

(Please pick either of the following options)

Doc 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: ( 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? (no)
• If yes, how is the feature documented? (docs )

[davidradl]

@reswqa as agreed, here are some doc changes as a follow on to #26437. Let me know if you want any changes.

[flinkbot]

CI report:

• 4f514b6 Azure: SUCCESS

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[reswqa]

Thanks for this PR, left some minor comments.

[davidradl]

@reswqa I have made the changes - Let's go to merge.

I notice #25128 provides Chinese translations - you might want to look at that one atfer this has been merged.

I suggest I backport this to v2 and 1.20 WDYT?