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

PR Close #21423

aio/content/examples/rx-library/src/error-handling.ts

@@ -0,0 +1,26 @@
+
+import { Observable } from 'rxjs/Observable';
+import 'rxjs/add/observable/of';
+
+// #docregion
+
+import { ajax } from 'rxjs/observable/dom/ajax';
+import { map, catchError } from 'rxjs/operators';
+// Return "response" from the API. If an error happens,
+// return an empty array.
+const apiData = ajax('/api/data').pipe(
+  map(res => {
+    if (!res.response) {
+      throw new Error('Value expected!');
+    }
+    return res.response;
+  }),
+  catchError(err => Observable.of([]))
+);
+
+apiData.subscribe({
+  next(x) { console.log('data: ', x); },
+  error(err) { console.log('errors already caught... will not run'); }
+});
+
+// #enddocregion