Update changelog and migration guide to suggest adapting changes from the new cookiecutter.

Author: jasongrout

docs/source/changelog.md

@@ -7,6 +7,10 @@ This is a summary of changes in ipywidgets releases. For more detailed informati
 To see the full list of pull requests and issues, see the [8.0 milestone](https://github.com/jupyter-widgets/ipywidgets/milestone/30?closed=1) on GitHub, or the 
 [full list of changes since 7.x](https://github.com/jupyter-widgets/ipywidgets/compare/7.x...master).
 
+See the [user migration guide](./user_migration_guides.md) for suggestions about migrating your code that uses ipywidgets to 8.0.
+
+If you author a custom widget, please see the [migration guide](./migration_guides.md) for suggestions about migrating your widget to support ipywidgets 8.
+
 ### Users
 
 Here are some highlights of user-visible changes in ipywidgets 8.0.
@@ -117,7 +121,7 @@ We have made it easier to load widgets from content delivery networks.
 
 - The default CDN is changed from unpkg to jsDelivr ([#3121](https://github.com/jupyter-widgets/ipywidgets/pull/3121), [#1627](https://github.com/jupyter-widgets/ipywidgets/issues/1627))
 - You can use the `data-jupyter-widgets-cdn-only` attribute to load modules only from CDN ([#2792](https://github.com/jupyter-widgets/ipywidgets/pull/2792), [#2786](https://github.com/jupyter-widgets/ipywidgets/issues/2786))
-- We have updated the webpack public path settings so the HTMLManager and the Jupyter Notebook extensions pull assets from wherever they are loaded, rather than only from CDN. [#3464](https://github.com/jupyter-widgets/ipywidgets/pull/3464), [#3508](https://github.com/jupyter-widgets/ipywidgets/pull/3508)
+- We have updated the webpack public path settings so the HTMLManager and the Jupyter Notebook extensions pull assets from wherever they are loaded, rather than only from CDN. If you author a custom widget, we highly encourage you to apply similar changes to your widget by adapting the changes at https://github.com/jupyter-widgets/widget-cookiecutter/pull/103/files. [#3464](https://github.com/jupyter-widgets/ipywidgets/pull/3464), [#3508](https://github.com/jupyter-widgets/ipywidgets/pull/3508)
 
 
 #### Other changes

docs/source/migration_guides.md

@@ -27,6 +27,8 @@ than one that contains widgets that you instantiated with ipywidgets 7.x.
 For a summarized list of relevant changes, please consult the "Developers" section of the
 [changelog](./changelog).
 
+You may consider updating your widget by generating a new widget from the cookiecutter at https://github.com/jupyter-widgets/widget-cookiecutter and adapting the changes to your widget. The widget cookiecutter has been updated to use best practices in Python packaging and Jupyter Widget infrastructure.
+
 ### Updating setup.py
 
 Start by updating the dependency in your `setup.py` or `setup.cfg` to support 8.x.
@@ -68,6 +70,10 @@ The ``ManagerBase`` class has been split into an interface type `IWidgetManager`
 + "@jupyter-widgets/base-manager": "^1",
 ```
 
+### Updating the AMD module logic
+
+We highly encourage you to update your widget's logic around generating AMD modules for the CDN with changes similar to those at https://github.com/jupyter-widgets/widget-cookiecutter/pull/103/files. These changes allow your AMD module to be hosted anywhere, rather than hardcoding the `unpkg.com` CDN, and they remove the differences between the AMD module generated for the notebook extension and the AMD module generated for the CDN.
+
 ### Updating the client-side code
 
 #### Phosphor -> Lumino