Simplify Hello World apps and update docs (#28)

* Simplify Hello World apps
* Updated docs

docs/app-lifecycle.md

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+# Application Lifecycle
+
+The app goes through these states:
+
+![app lifecycle state diagram](pics/app-lifecycle.png)
+
+Let's look at a scenario where an app launches another app:
+
+1. `first` app starts: `first.on_create()` -> `first.on_show()`
+2. `second` app starts: `first.on_hide()` -> `second.on_create()` -> `second.on_show()`
+3. `second` app stops: `second.on_hide()` -> `second.on_destroy()` -> `first.on_show()`
+4. `first` app stops: `first.on_hide()` -> `first.on_destroy()`

docs/app-lifecycle.puml

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+@startuml
+[*] --> on_create : app is started
+on_create --> on_show : app becomes visible
+on_show --> on_hide : app is no longer visible
+on_hide --> on_destroy : app is being closed
+on_destroy --> [*]
+skinparam ranksep 25
+skinparam padding 2
+@enduml