[docs] Add documentation for lookup.insert-if-not-exists feature

[platinumhamburg]

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• I searched in the issues and found nothing similar.

Description

The lookup.insert-if-not-exists feature is an important capability that combines Primary Key tables with auto-increment columns to automatically build dictionary tables during Flink Lookup Joins. However, the current documentation only has a one-line description in the Connector Options table, with no dedicated section, usage instructions, or examples.

Willingness to contribute

• I'm willing to submit a PR!

[wuchong]

@platinumhamburg could you open a pull request for release-0.9 branch as well?

[platinumhamburg]

@platinumhamburg could you open a pull request for release-0.9 branch as well?

Yes, I opened a PR #3037 for release-0.9, PTAL.