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could get out-of-date or contain compile errors without as noticing.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/practical-observable-usage/example-config.json

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