fix(docs-app): get rid of document.write for inserting scripts #15396

Since Chrome 54, scripts inserted via document.write generate a warning and might be blocked on slow networks (see this article for more info).

Since we are using document.write for scipts in our docs app, it might break on slow networks.

The warning message is:

A Parser-blocking, cross-origin script, https://ajax.googleapis.com/ajax/libs/angularjs/1.5.8/angular-min.js, is invoked via document.write. This may be blocked by the browser if the device has poor network connectivity.