Document: feat(sink): introduce exactly once iceberg sink

[neverchanje]

This issue tracks the documentation update needed for the merged PR #19771.

Source PR URL: risingwavelabs/risingwave#19771
Source PR Merged At: 2025-03-27T07:38:29Z

If it is a major improvement that deserves a new page or a new section in the documentation, please check if we should label it as an experiment feature.