docs: describe changes to the extensions system

docs/en/UI/Angular/Migration-Guide-v3.md

@@ -358,6 +358,15 @@ Once you bind the injected `ListService` instance through `NgxDatatableListDirec
 **Important Note:** The `abp-table` is not removed, but is deprecated and will be removed in the future. Please consider switching to ngx-datatable.
 
 
+### Extensions System [COMMERCIAL]
+
+The extensions system is open sourced now and is publicly available from `@abp/ng.theme.shared/extensions` package instead of `@volo/abp.commercial.ng.ui`. Also, according to the new structure of config packages, the configuration is given through `forLazy` static methods as described above.
+
+#### What to Do When Migrating?
+
+If you have never used the extensions system before, you do not have to do anything. If you have, then please check the documentation again to see what changed. Extension system itself works the same as before. The only changes are the package you import from and the static method and the module you pass your contributors to.
+
+
 ### Lepton Theme Logos [COMMERCIAL]
 
 In ABP v2.x, Lepton had one light and one dark logo per color theme. We have realized we could make it work with only one light and one dark logo. So, we have changed how Lepton looks up logo images and now you just need to have a `logo-light.png` and a `logo-dark.png` in your project.