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

PR Close #21423

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

@@ -0,0 +1,21 @@
+
+import { Observable } from 'rxjs/Observable';
+import 'rxjs/add/observable/of';
+
+// #docregion
+
+import { map } from 'rxjs/operators';
+
+const nums = Observable.of(1, 2, 3);
+
+const squareValues = map((val: number) => val * val);
+const squaredNums = squareValues(nums);
+
+squaredNums.subscribe(x => console.log(x));
+
+// Logs
+// 1
+// 4
+// 9
+
+// #enddocregion