Add Flink 2.2 Delta Join tests and documentation

[wuchong]

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Motivation

#2167 supports Flink 2.2 in Fluss and added some tests in Flink22TableSourceITCase. However, the tests are very limited:

• use first_row fluss table, rather than normal primary key tables (may need to enable table.delete.behavior=ignore)
• join on primary keys, rather than bucket key
• innser join, rather than left join or right join

We should also update the documentation with new examples and new limitations. https://fluss.apache.org/docs/next/engine-flink/delta-joins/

Solution

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Anything else?

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Willingness to contribute

• I'm willing to submit a PR!

[wuchong]

@xuyangzhong could you help with this? cc @loserwang1024