Add documentation for shake effect

Author: jsteinbeck

docs/reference/elements/shake.md

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+
+# [command] shake
+
+Shakes a displayable asset. This is similar to the punch effects known from Ren'Py.
+
+
+## Usage
+
+    . shake @my_image, :200, times 4
+
+
+## Attributes
+
+* **asset/@[value]:** The asset's internal name.
+* **duration:** The duration for the effect to last in milliseconds. Default: 275.
+* **times:** The number of times the asset shakes back and forth.
+* **ifvar:** See [conditionals](conditionals.md).
+* **ifvalue:** See [conditionals](conditionals.md).
+* **ifnot:** See [conditionals](conditionals.md).
+
+
+## Parents
+
+* [scene](scene.md)
+
+
+## Children
+
+* none
+
+
+## Related Assets
+
+* [imagepack](imagepack.md)
+* [textbox](textbox.md)
+* [curtain](curtain.md)
+
+
+## Related Commands
+
+ * [flicker](flicker.md)
+