[FLINK-40131][docs] Document LATERAL SNAPSHOT join

[fhueske]

What is the purpose of the change

Add documentation for FLIP-579: LATERAL SNAPSHOT join

Brief change log

Add docs for FLIP-579: LATERAL SNAPSHOT join

Verifying this change

docs only change

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? n/a

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Generated-by: Claude Code 2.1.200 (Opus 4.8)

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CI report:

• bdde889 Azure: FAILURE

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