Finish up Animator docs first pass

docs/examples/guide/animator/animation04.py

@@ -0,0 +1,29 @@
+from rich.console import RenderableType
+
+from textual.app import App, ComposeResult
+from textual.reactive import reactive
+from textual.widget import Widget
+
+
+class ValueBox(Widget):
+    value = reactive(0.0)
+
+    def render(self) -> RenderableType:
+        return str(self.value)
+
+
+class AnimationApp(App):
+    def compose(self) -> ComposeResult:
+        self.box = ValueBox()
+        self.box.styles.background = "red"
+        self.box.styles.color = "black"
+        self.box.styles.padding = (1, 2)
+        yield self.box
+
+    async def on_mount(self):
+        self.box.animate("value", value=100.0, duration=100.0, easing="linear")
+
+
+if __name__ == "__main__":
+    app = AnimationApp()
+    app.run()