[FLINK-39820][docs] Update delta join documentation with new features in 2.3

[xuyangzhong]

What is the purpose of the change

This pull request updates the delta join documentation to reflect new capabilities introduced in Flink 2.3, including cascaded delta join, lookup join after delta join, non-deterministic function support, and the immutable columns constraint.

Brief change log

• Added cascaded delta join, lookup join after delta join, and non-deterministic function support to the "Supported Features" list
• Updated the limitation on non-deterministic functions to clarify it only applies between the source and the delta join
• Changed limitations 4 and 5 to reference "upsert key" instead of "primary key", with a footnote explaining the immutable columns constraint
• Added a hint block describing the immutable columns constraint and its relationship to upsert key enrichment

Verifying this change

None for doc.

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?

[xuyangzhong]

@lincoln-lil @Au-Miner Can you take a look if you have time?

[flinkbot]

CI report:

• 982a42d Azure: FAILURE

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