app basics and doc structure

docs/examples/app/event01.py

@@ -0,0 +1,30 @@
+from textual.app import App
+from textual import events
+
+
+class EventApp(App):
+
+    COLORS = [
+        "white",
+        "maroon",
+        "red",
+        "purple",
+        "fuchsia",
+        "olive",
+        "yellow",
+        "navy",
+        "teal",
+        "aqua",
+    ]
+
+    def on_mount(self) -> None:
+        self.styles.background = "darkblue"
+
+    def on_key(self, event: events.Key) -> None:
+        if event.key.isdecimal():
+            self.styles.background = self.COLORS[int(event.key)]
+
+
+app = EventApp()
+if __name__ == "__main__":
+    app.run()