docs(aio): add Observable and Rx docs (#21423)

PR Close #21423

aio/content/examples/observables/src/geolocation.ts

@@ -0,0 +1,32 @@
+import { Observable } from 'rxjs/Observable';
+
+// #docregion
+
+// Create an Observable that will start listening to geolocation updates
+// when a consumer subscribes.
+const locations = new Observable((observer) => {
+  // Get the next and error callbacks. These will be passed in when
+  // the consumer subscribes.
+  const {next, error} = observer;
+  let watchId;
+
+  // Simple geolocation API check provides values to publish
+  if ('geolocation' in navigator) {
+    watchId = navigator.geolocation.watchPosition(next, error);
+  } else {
+    error('Geolocation not available');
+  }
+
+  // When the consumer unsubscribes, clean up data ready for next subscription.
+  return {unsubscribe() { navigator.geolocation.clearWatch(watchId); }};
+});
+
+// Call subscribe() to start listening for updates.
+const locationsSubscription = locations.subscribe({
+  next(position) { console.log('Current Position: ', position); },
+  error(msg) { console.log('Error Getting Location: ', msg); }
+});
+
+// Stop listening for location after 10 seconds
+setTimeout(() => { locationsSubscription.unsubscribe(); }, 10000);
+// #enddocregion