[docs] Adding Confluent to vendors list

[gunnarmorling]

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• e20efb7 Azure: SUCCESS

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[MartijnVisser]

I didn't notice you had this one opened @gunnarmorling - Can you check out https://flink.apache.org/how-to-contribute/code-style-and-quality-pull-requests/

1. The PR title should start with [FLINK-XXXX] so in this case that would be FLINK-37967, which is now assigned to you
2. The commit should also have started with [FLINK-37967], followed by the component. In this case that would be [FLINK-37697][Documentation] Adding Confluent to vendors list
3. You've only added it to the English docs, but we also need to have it changed in the Chinese version. See https://flink.apache.org/how-to-contribute/contribute-documentation/#chinese-documentation-translation

Can you include the addition in the Chinese docs?