Update crash course

docs/tut/crash-course/11-actions.md

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+# Actions
+
+Actions in Vide are special callbacks that you can pass along with properties,
+which will be called when those properties are being processed with the instance
+being assigned to, allowing you to run custom code.
+
+```lua
+local action = vide.action
+```
+
+```lua
+create "TextLabel" {
+    Text = "test",
+
+    action(function(instance)
+        print(instance.Text)
+    end)
+}
+
+-- will print "test"
+```
+
+Actions can be wrapped with functions to re-use specific behaviors. Below is
+an example of an action used to listen for property changes:
+
+```lua
+local action = vide.action
+local cleanup = vide.cleanup
+
+local function changed(property: string, callback: (new) -> ())
+    return action(function(instance)
+        local con = instance:GetPropertyChangedSignal(property):Connect(function()
+            callback(instance[property])
+        end)
+
+        -- remember to clean up the connection when the reactive scope the action
+        -- is ran in is destroyed, so the instance can be garbage collected
+        cleanup(function()
+            con:Disconnect()
+        end)
+    end)
+end
+
+local output = source ""
+
+local instance = create "TextBox" {
+    changed("Text", output)
+}
+
+instance.Text = "foo"
+
+print(output()) -- "foo"
+```
+
+The source `output` will be updated with the new property value any time it is
+changed externally.