[Experimental] Start Migration Guide (#4060)

Covers the Manager, ThreeDScene, Camera, and Animations

Co-authored-by: Francisco Manríquez Novoa <49853152+chopan050@users.noreply.github.com>

---------

Co-authored-by: Francisco Manríquez Novoa <49853152+chopan050@users.noreply.github.com>

docs/source/changelog.rst

@@ -9,6 +9,7 @@ releases since v0.18.0) are documented on our
 
 .. toctree::
 
+    changelog/experimental
     changelog/0.18.0-changelog
     changelog/0.17.3-changelog
     changelog/0.17.2-changelog

docs/source/changelog/experimental.md

@@ -0,0 +1,97 @@
+# Migrating from v0.19.0 to v0.20.0
+
+This constitutes a list of all the changes needed to migrate your code
+to work with the latest version of Manim
+
+## Manager
+If you ever used `Scene.render`, you must replace it with {class}`.Manager`.
+
+Original code:
+```py
+scene = SceneClass()
+scene.render()
+```
+should be changed to:
+```py
+manager = Manager(SceneClass)
+manager.render()
+```
+
+If you are a plugin author that subclasses `Scene` and changed `Scene.render`, you should migrate
+your code to use the specific public methods on {class}`.Manager` instead.
+
+## ThreeDScene and Camera
+`ThreeDScene` has been completely removed, and all of its functionality has been replaced
+with methods on {class}`.Camera`, which can be accessed via {attr}`.Scene.camera`.
+
+For example, the following code
+```py
+class MyScene(ThreeDScene):
+    def construct(self):
+        t = Text("Hello")
+        self.add_fixed_in_frame_mobjects(t)
+        self.begin_ambient_camera_rotation()
+        self.wait(3)
+```
+should be changed to
+```py
+# change ThreeDScene -> Scene
+class MyScene(Scene):
+    def construct(self):
+        t = Text("Hello")
+        # add_fixed_in_frame_mobjects() no longer exists.
+        # Now you must use Mobject.fix_in_frame() manually for each Mobject.
+        t.fix_in_frame()
+        self.add(t)
+
+        # access the method on the camera
+        self.camera.begin_ambient_rotation()
+        self.add(self.camera)
+        self.wait(3)
+```
+
+## Animation
+`Animation.interpolate_mobject` has been combined into `Animation.interpolate`.
+
+Methods `Animation._setup_scene` and `Animation.clean_up_from_scene` have been removed
+in favor of `Animation.begin` and `Animation.finish`. If you need to access the scene,
+you can use a simple buffer to communicate. Note that this buffer cannot access
+methods on the {class}`.Scene`, but can only do basic actions like {meth}`.Scene.add`,
+{meth}`.Scene.remove`, and {meth}`.Scene.replace`.
+
+For example, the following code:
+```py
+class MyAnimation(Animation):
+    def begin(self) -> None:
+        self._sqrs = VGroup(Square())
+
+    def _setup_scene(self, scene: Scene) -> None:
+        scene.add(self._sqr)
+        self.scene = scene
+
+    def interpolate_mobject(self, alpha: float) -> None:
+        sqr = Square().move_to((alpha, 0, 0))
+        self._sqrs.add(sqr)
+        self.scene.add(sqr)
+
+    def clean_up_from_scene(self, scene: Scene) -> None:
+        scene.remove(self._sqrs)
+```
+
+should be changed to
+```py
+class MyAnimation(Animation):
+    def begin(self) -> None:
+        self._sqrs = VGroup(Square())
+        self.buffer.add(self._sqrs)
+
+    def interpolate(self, alpha: float) -> None:
+        sqr = Square().move_to((alpha, 0, 0))
+        self._sqrs.add(sqr)
+        self.buffer.add(sqr)
+        # tell the scene to empty the buffer
+        self.apply_buffer = True
+
+    def finish(self) -> None:
+        self.buffer.remove(self._sqrs)
+```