Mention prebuilt extensions in README and docs (#10604)

README.md

@@ -31,7 +31,12 @@ powerful user interface.
 JupyterLab will eventually replace the classic Jupyter Notebook.
 
 JupyterLab can be extended using [npm](https://www.npmjs.com/) packages
-that use our public APIs. To find JupyterLab extensions, search for the npm keyword [jupyterlab-extension](https://www.npmjs.com/search?q=keywords:jupyterlab-extension) or the GitHub topic [jupyterlab-extension](https://github.com/topics/jupyterlab-extension). To learn more about extensions, see the [user documentation](https://jupyterlab.readthedocs.io/en/stable/user/extensions.html).
+that use our public APIs. The _prebuilt_ extensions can be distributed
+via [PyPI](https://pypi.org/search/?q=jupyterlab&o=-created&c=Framework+%3A%3A+Jupyter),
+conda, and other package managers. The _source_ extensions can be installed
+directly from npm (search for [jupyterlab-extension](https://www.npmjs.com/search?q=keywords:jupyterlab-extension)) but require additional build step.
+You can also find JupyterLab extensions exploring GitHub topic [jupyterlab-extension](https://github.com/topics/jupyterlab-extension).
+To learn more about extensions, see the [user documentation](https://jupyterlab.readthedocs.io/en/stable/user/extensions.html).
 
 The current JupyterLab releases are suitable for general
 usage, and the extension APIs will continue to

docs/source/extension/extension_migration.rst

@@ -105,7 +105,7 @@ 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.
+``@jupyterlab/builder`` is a package required to build the extension as a federated (prebuilt) 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

docs/source/user/directories.rst

@@ -10,7 +10,8 @@ JupyterLab manages several different locations for its data. These locations are
 
 JupyterLab also honors **LabConfig Directories** directories for configuration data from the ``labconfig`` subdirectories of the Jupyter ``config`` directories in the Jupyter path hierarchy.
 
-Additionally, JupyterLab can load dynamic federated extensions, i.e., extensions that bundle their dependencies, from the ``labextensions`` subdirectories of the Jupyter ``data`` directories. 
+Additionally, JupyterLab can load dynamic federated (prebuilt) extensions, i.e., extensions that
+bundle their dependencies, from the ``labextensions`` subdirectories of the Jupyter ``data`` directories.
 
 See the locations of these Jupyter config paths by running ``jupyter --path``.