build(aio): move doc-gen stuff from angular.io (#14097)

docs/content/cheatsheet/bootstrapping.md

@@ -0,0 +1,18 @@
+@cheatsheetSection
+Bootstrapping
+@cheatsheetIndex 0
+@description
+{@target ts}`import { platformBrowserDynamic } from '@angular/platform-browser-dynamic';`{@endtarget}
+{@target js}Available from the `ng.platformBrowserDynamic` namespace{@endtarget}
+
+@cheatsheetItem
+syntax(ts):
+`platformBrowserDynamic().bootstrapModule(AppModule);`|`platformBrowserDynamic().bootstrapModule`
+syntax(js):
+`document.addEventListener('DOMContentLoaded', function() {
+  ng.platformBrowserDynamic
+    .platformBrowserDynamic()
+    .bootstrapModule(app.AppModule);
+});`|`platformBrowserDynamic().bootstrapModule`
+description:
+Bootstraps the app, using the root component from the specified `NgModule`. {@target js}Must be wrapped in the event listener to fire when the page loads.{@endtarget}