Fix bad custom animation example

The custom animation example suggests that a stack-allocated
AnimationImplementation variable can be used, but this is incorrect.
The variable is not copied and so must remain valid while the
animation runs.

For brevity, I have used a static qualifier to give the variable
global lifespan without making it an explicit global variable.

I have also added a note about the lifetime of the
AnimationImplementation variable.

Author: a1291762

source/_guides/graphics-and-animations/animations.md

@@ -187,7 +187,7 @@ animation_set_delay(animation, 1000);
 animation_set_duration(animation, 1000);
 
 // Create the AnimationImplementation
-const AnimationImplementation implementation = {
+static const AnimationImplementation implementation = {
   .setup = implementation_setup,
   .update = implementation_update,
   .teardown = implementation_teardown
@@ -198,6 +198,9 @@ animation_set_implementation(animation, &implementation);
 animation_schedule(animation);
 ```
 
+Note that the AnimationImplementation variable is not copied and must remain
+valid while the animation is running.
+
 The output of the example above will look like the snippet shown below (edited
 for brevity). Note the effect of the easing ``AnimationCurve`` on the progress
 value: