test(docs-infra): ensure RxJS-related docs examples can compile without errors (#34063)

Previously, some RxJS-related examples (which are not proper Angular apps) were not
tested on CI as part of the `example-e2e` npm script. This meant that the examples
could get out-of-date or contain compile errors without as noticing.

This commit ensures that the `example-e2e` script picks up these examples and checks
that they compile successfully.

Partly addresses #28017.

PR Close #34063

aio/content/examples/observables-in-angular/e2e/src/app.e2e-spec.ts

@@ -0,0 +1,9 @@
+/*
+ * This example project is special in that it is not a cli app. To run tests appropriate for this
+ * project, the test command is overwritten in `aio/content/examples/observables-in-angular/example-config.json`.
+ *
+ * This is an empty placeholder file to ensure that `aio/tools/examples/run-example-e2e.js` runs
+ * tests for this project.
+ *
+ * TODO: Fix our infrastructure/tooling, so that this hack is not necessary.
+ */

aio/content/examples/observables-in-angular/example-config.json

@@ -0,0 +1,8 @@
+{
+  "e2e": [
+    {
+      "cmd": "yarn",
+      "args": [ "tsc", "--project", "./" ]
+    }
+  ]
+}

aio/content/examples/observables-in-angular/src/main.ts

@@ -39,9 +39,9 @@ export class ZippyComponent {
        Time: {{ time | async }}</div>`
 })
 export class AsyncObservablePipeComponent {
-  time = new Observable(observer =>
-    setInterval(() => observer.next(new Date().toString()), 1000)
-  );
+  time = new Observable<string>(observer => {
+    setInterval(() => observer.next(new Date().toString()), 1000);
+  });
 }
 
 // #enddocregion pipe