more docs and compute example

docs/examples/guide/reactivity/watch01.py

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+from textual.app import App, ComposeResult
+from textual.color import Color, ColorParseError
+from textual.containers import Grid
+from textual.reactive import reactive
+from textual.widgets import Input, Static
+
+
+class WatchApp(App):
+    CSS_PATH = "watch01.css"
+
+    color = reactive(Color.parse("transparent"))  # (1)!
+
+    def compose(self) -> ComposeResult:
+        yield Input(placeholder="Enter a color")
+        yield Grid(Static(id="old"), Static(id="new"), id="colors")
+
+    def watch_color(self, old_color: Color, new_color: Color) -> None:  # (2)!
+        self.query_one("#old").styles.background = old_color
+        self.query_one("#new").styles.background = new_color
+
+    def on_input_submitted(self, event: Input.Submitted) -> None:
+        try:
+            input_color = Color.parse(event.value)
+        except ColorParseError:
+            pass
+        else:
+            self.query_one(Input).value = ""
+            self.color = input_color  # (3)!
+
+
+if __name__ == "__main__":
+    app = WatchApp()
+    app.run()