docs(animimg) Add missing animation image page (#2609)

* docs(animimg) Add missing animation image page

Skeleton page for the new `animimg` object.
It needs more review and updating, but this can serve as a start.

* docs(examples) Add index.rst for animation image

Add index.rst for animation image to include the example in the documentation of the widget.

* Update animimg.md

examples/widgets/animimg/index.rst

@@ -0,0 +1,7 @@
+
+Simple Animation Image 
+""""""""""""""""
+
+.. lv_example:: widgets/animimg/lv_example_animimg_1.c 
+  :language: c
+  :description: A simple example to demonstrate the use of an animation image.