docs(service-worker): fix the `service-worker-getting-started` docs example (#28020)

Changes:
- Change the project type to `service-worker`, so that it gets
  appropriate `package.json` (with `@angular/service-worker` dependency)
  and `angular.json` (with `serviceWorker: true` in production config).
- Move `ngsw-config.json` to the correct directory.
- Specify custom test commands for aio's `yarn example-e2e` to also
  verify that the ServiceWorker bits are set up correctly.

PR Close #28020

aio/content/examples/service-worker-getting-started/example-config.json

@@ -0,0 +1,10 @@
+{
+  "projectType": "service-worker",
+  "e2e": [
+    {"cmd": "yarn", "args": ["e2e", "--no-webdriver-update"]},
+    {"cmd": "yarn", "args": ["build", "--prod"]},
+    {"cmd": "node", "args": ["--eval", "assert(fs.existsSync('./dist/ngsw.json'), 'ngsw.json is missing')"]},
+    {"cmd": "node", "args": ["--eval", "assert(fs.existsSync('./dist/ngsw-worker.js'), 'ngsw-worker.js is missing')"]},
+    {"cmd": "node", "args": ["--eval", "assert(require('./package.json').dependencies['@angular/service-worker'], '@angular/service-worker is missing')"]}
+  ]
+}