Flink: Add documentation for Dynamic Iceberg Sink

[mxm]

Now that we have finalized and merged the design of the Flink Dynamic Iceberg Sink (#12424), we should add documentation for the upcoming 1.10.0 release.

[mxm]

We should add documentation to the Flink Write page: https://iceberg.apache.org/docs/nightly/flink-writes/

[github-actions]

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[mxm]

This has been addressed in #13608.