Update app-model documentation (#325)

# Description

Updates the app-model documentation:

* Fleshes out existing information and adds napari specific information.
* Add guide on testing app-model
* Add details on app-model vs action manager implementation differences

This depends on napari 4991 as I link to a file in the repo that is
created in that PR.

---------

Co-authored-by: Peter Sobolewski <76622105+psobolewskiPhD@users.noreply.github.com>
Co-authored-by: Draga Doncila Pop <17995243+DragaDoncila@users.noreply.github.com>

docs/conf.py

@@ -161,6 +161,10 @@
         'https://pyapp-kit.github.io/magicgui/',
         'https://pyapp-kit.github.io/magicgui/objects.inv',
     ],
+    'app-model': [
+        'https://app-model--142.org.readthedocs.build/en/142/',
+        'https://app-model--142.org.readthedocs.build/en/142/objects.inv',
+    ],
 }
 
 myst_enable_extensions = [