Mention jupyterlab/builder dev dependency

docs/source/developer/extension_migration_2_3.rst

@@ -98,6 +98,14 @@ The upgrade script also updates the dependencies in ``package.json`` to the ``^3
 
 On the diff above, we see that additional development scripts are also added, as they are used by the new extension system workflow.
 
+The diff also shows the new ``@jupyterlab/builder`` as a ``devDependency``.
+``@jupyterlab/builder`` is a package required to build the extension as a federated extension.
+It hides away internal dependencies such as ``webpack``, and produces the assets that can then be distributed as part of a Python package.
+
+Extension developers do not need to interact with ``@jupyterlab/builder`` directly, but instead can use the
+``jupyter labextension build`` command. This command is run automatically as part of the ``build`` script
+(``jlpm run build``).
+
 Publishing the extension to PyPI and conda-forge
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