what’s new for coalescing stream output

docs/source/whatsnew/pr/coalesce-streams.rst

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+- Consecutive stream (stdout/stderr) output is merged into a single output
+  in the notebook document.
+  Previously, all output messages were preserved as separate output fields in the JSON.
+  Now, the same merge is applied to the stored output as the displayed output,
+  improving document load time for notebooks with many small outputs.