Add Homotopy example for waving square (#4100)

* Add missing changes for homotopy example

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manim/animation/movement.py

@@ -44,6 +44,26 @@ class Homotopy(Animation):
         Keyword arguments propagated to :meth:`.Mobject.apply_function`.
     kwargs
         Further keyword arguments passed to the parent class.
+
+    Examples
+    --------
+
+    .. manim:: HomotopyExample
+
+        class HomotopyExample(Scene):
+            def construct(self):
+                square = Square()
+
+                def homotopy(x, y, z, t):
+                    if t <= 0.25:
+                        progress = t / 0.25
+                        return (x, y + progress * 0.2 * np.sin(x), z)
+                    else:
+                        wave_progress = (t - 0.25) / 0.75
+                        return (x, y + 0.2 * np.sin(x + 10 * wave_progress), z)
+
+                self.play(Homotopy(homotopy, square, rate_func= linear, run_time=2))
+
     """
 
     def __init__(