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

PR Close #21423

aio/content/examples/rx-library/src/operators.2.ts

@@ -0,0 +1,18 @@
+import { Observable } from 'rxjs/Observable';
+import 'rxjs/add/observable/of';
+
+// #docregion
+
+import { filter } from 'rxjs/operators/filter';
+import { map } from 'rxjs/operators/map';
+
+const squareOdd = Observable.of(1, 2, 3, 4, 5)
+  .pipe(
+    filter(n => n % 2),
+    map(n => n * n)
+  );
+
+// Subscribe to get values
+squareOdd.subscribe(x => console.log(x));
+
+// #enddocregion