Add animation documentation and asset folder

Animation documentation is not yet complete.
Asset folder contains renders from code snipples.

Update constants.rst to include a ref tag
Update index to include the animation doc

docs/source/animation.rst

@@ -0,0 +1,160 @@
+Animation
+=========
+
+
+
+The simplest of which is ``Scene.add``. The object appears on the first frame
+without any animation::
+
+  class NoAnimation(Scene):
+      def construct(self):
+          square = Square()
+          self.add(square))
+
+Animation are used in conjunction with ``scene.Play``
+
+Fade
+----
+
+FadeIn :download:`/assets/AnimationFadeIn.mp4`::
+
+  class AnimationFadeIn(Scene):
+      def construct(self):
+          square = Square()
+
+          anno = TextMobject("Fade In")
+          anno.shift(2 * DOWN)
+          self.add(anno)
+          self.play(FadeIn(square))
+
+
+
+FadeOut :download:`/assets/AnimationFadeOut.mp4`::
+
+  class AnimationFadeOut(Scene):
+      def construct(self):
+          square = Square()
+
+          anno = TextMobject("Fade Out")
+          anno.shift(2 * DOWN)
+          self.add(anno)
+          self.add(square)
+          self.play(FadeOut(square))
+
+
+
+FadeInFrom :download:`/assets/AnimationFadeInFrom.mp4`::
+
+  class AnimationFadeInFrom(Scene):
+      def construct(self):
+          square = Square()
+          for label, edge in zip(
+              ["LEFT", "RIGHT", "UP", "DOWN"], [LEFT, RIGHT, UP, DOWN]
+          ):
+              anno = TextMobject(f"Fade In from {label}")
+              anno.shift(2 * DOWN)
+              self.add(anno)
+
+              self.play(FadeInFrom(square, edge))
+              self.remove(anno, square)
+
+
+
+FadeOutAndShift :download:`assets/AnimationFadeOutAndShift.mp4`::
+
+  class AnimationFadeOutAndShift(Scene):
+      def construct(self):
+          square = Square()
+          for label, edge in zip(
+              ["LEFT", "RIGHT", "UP", "DOWN"], [LEFT, RIGHT, UP, DOWN]
+          ):
+              anno = TextMobject(f"Fade Out and shift {label}")
+              anno.shift(2 * DOWN)
+              self.add(anno)
+
+              self.play(FadeOutAndShift(square, edge))
+              self.remove(anno, square)
+
+
+
+FadeInFromLarge :download:`assets/AnimationFadeInFromLarge.mp4`::
+
+  class AnimationFadeInFromLarge(Scene):
+      def construct(self):
+          square = Square()
+
+          for factor in [0.1, 0.5, 0.8, 1, 2, 5]:
+              anno = TextMobject(f"Fade In from large scale\_factor={factor}")
+              anno.shift(2 * DOWN)
+              self.add(anno)
+
+              self.play(FadeInFromLarge(square, scale_factor=factor))
+              self.remove(anno, square)
+
+
+
+FadeInFromPoint :download:`assets/AnimationFadeInFromPoint.mp4`::
+
+  class AnimationFadeInFromPoint(Scene):
+      def construct(self):
+          square = Square()
+          for i in range(-6, 7, 2):
+              anno = TextMobject(f"Fade In from point {i}")
+              anno.shift(2 * DOWN)
+              self.add(anno)
+              self.play(FadeInFromPoint(square, point=i))
+              self.remove(anno, square)
+
+
+
+Grow
+----
+
+GrowFromEdge :download:`assets/AnimationGrowFromEdge.mp4`::
+
+  class AnimationGrowFromEdge(Scene):
+      def construct(self):
+
+          for label, edge in zip(
+              ["LEFT", "RIGHT", "UP", "DOWN"], [LEFT, RIGHT, UP, DOWN]
+          ):
+              anno = TextMobject(f"Grow from {label} edge")
+              anno.shift(2 * DOWN)
+              self.add(anno)
+              square = Square()
+              self.play(GrowFromEdge(square, edge))
+              self.remove(anno, square)
+
+
+
+GrowFromCenter :download:`assets/AnimationGrowFromCenter.mp4`::
+
+  class AnimationGrowFromCenter(Scene):
+      def construct(self):
+          square = Square()
+
+          anno = TextMobject("Grow from center")
+          anno.shift(2 * DOWN)
+          self.add(anno)
+
+          self.play(GrowFromCenter(square))
+
+
+
+
+Diagonal Directions
+-------------------
+
+You can combine cardinal directions to form diagonal animations :download:`assets/AnimationFadeInFromDiagnal.mp4`::
+
+  class AnimationFadeInFromDiagnal(Scene):
+      def construct(self):
+          square = Square()
+          for diag in [UP + LEFT, UP + RIGHT, DOWN + LEFT, DOWN + RIGHT]:
+              self.play(FadeInFrom(square, diag))
+
+
+
+.. note::
+    You can also use the abbreviated forms like ``UL, UR, DL, DR``.
+    See :ref:`ref-directions`.

docs/source/constants.rst

@@ -75,6 +75,8 @@ Render setting presets
     LOW_QUALITY_CAMERA_CONFIG
             854x480 @ 15fps. # Used when ``-l`` flag is passed
 
+.. _ref-directions:
+
 Coordinates
 -----------
 

docs/source/index.rst

@@ -17,8 +17,9 @@ a pull request there.
     about
     installation/index
     getting_started/index
+    animation
     constants
-    
+
 
 Indices and tables
 ==================