[FLINK-37799][model][docs] Add document for OpenAI Model Function

[yunfengzhou-hub]

What is the purpose of the change

This PR is a continuation to #26652, adding document for the introduced OpenAI Model Function.

Brief change log

• Adds document for the OpenAI Model Function.

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: 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:

• 18640aadf02df897a3785af731d02d45880edb12 Azure: SUCCESS

Bot commands

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• @flinkbot run azure re-run the last Azure build

[yunfengzhou-hub]

Hi @fsk119 @lihaosky could you please help review this PR?