rename "App, Appbuilder, and World" => "Apps and Worlds"

content/learn/book/getting-started/apps-worlds.md

@@ -0,0 +1,34 @@
++++
+title = "Apps and Worlds"
+[extra]
+weight = 3
+status = 'hidden'
++++
+
+Your entire game is stored inside a Bevy [`App`], which is responsible for running the main game loop and storing the main [`World`].
+The [`World`] stores all of the App's state — [Entities], [Components], [Resources], and [Systems].
+
+[`App`]: https://docs.rs/bevy/latest/bevy/app/struct.App.html
+[`World`]: https://docs.rs/bevy/latest/bevy/ecs/world/struct.World.html
+[Entities]: /learn/book/ecs/entities-components/
+[Components]: /learn/book/ecs/entities-components/#working-with-components
+[Resources]: /learn/book/ecs/resources/
+[Systems]: /learn/book/ecs/systems-queries/
+
+## Apps
+
+{% todo() %}
+
+* explain what the app does
+* example `main()` function
+
+{% end %}
+
+## Worlds
+
+{% todo() %}
+
+* explain what the world does
+* vaguely explain how the world actually stores entities and components? maybe too early for that
+
+{% end %}