Add explainer gif (#2357)

The gif is not 100% done but close enough.

Also,  ignore the tmp/ folder. It’s used for code snippet testing (`tool/check-code.sh`).

Author: filiph

src/_assets/image/development/data-and-backend/state-mgmt/state-management-explainer.gif

@@ -10,11 +10,15 @@ _If you are already familiar with state management in reactive apps, you can
 skip this section, though you might want to review the [list of different 
 approaches](/docs/development/data-and-backend/state-mgmt/options)._
 
+{% asset development/data-and-backend/state-mgmt/state-management-explainer width="100%" alt="A short animated gif that shows the workings of a simple declarative state management system. This is explained in full in one of the following pages. Here it's just a decoration." %}
+
+{% comment %} 
+Source of the above animation tracked internally as b/122314402 
+{% endcomment %}
+
 As you explore Flutter, there comes a time when you need to share application 
 state between screens, across your app. There are many approaches you can take, 
 and many questions to think about.
 
-{% comment %} <!-- decorative gif animation to be added --> {% endcomment %}
-
 In the following pages, you will learn the basics of dealing with state in 
 Flutter apps.