test(docs-infra): add tests for universal docs example (#36483)

Previously, there were no tests for the `universal` docs example. This
meant that the project was not tested at all (not even ensuring that it
can be built successfully).

This commit adds e2e tests for the `universal` example (ported from
`toh-pt6` and cleaned up) and also verifies that the project can be
built successfully (including the server).

PR Close #36483

aio/content/examples/universal/example-config.json

@@ -1,3 +1,7 @@
 {
-  "projectType": "universal"
+  "projectType": "universal",
+  "e2e": [
+    {"cmd": "yarn", "args": ["e2e", "--prod", "--protractor-config=e2e/protractor-puppeteer.conf.js", "--no-webdriver-update", "--port={PORT}"]},
+    {"cmd": "yarn", "args": ["run", "build:ssr"]}
+  ]
 }