build(docs-infra): update docs examples package.json templates wrt core-js (#36143)

The `core-js` dependency is no longer included in `package.json` for
`cli`-type examples, but only for the `systemjs` ones. This commit
updates the `package.json` templates to reflect that (and also updates
the `npm-packages` guide accordingly).

PR Close #36143

aio/tools/example-zipper/customizer/package-json/base.json

@@ -13,7 +13,6 @@
     "@angular/router",
     "@angular/upgrade",
     "angular-in-memory-web-api",
-    "core-js",
     "rxjs",
     "zone.js"
   ],

aio/tools/example-zipper/customizer/package-json/systemjs.json

@@ -25,6 +25,7 @@
     "@angular/platform-browser-dynamic",
     "@angular/router",
     "@angular/upgrade",
+    "core-js",
     "rxjs",
     "systemjs",
     "tslib",