Previously there was a custom version of the testing app for Stackblitz that would run the tests in the browser.
With the web-container approach, this is no longer necessary since the tests can be run directly from the command line.
Fixes#44047
PR Close#44085
Content projection is an important part of Angular. However, it has
never been documented in the Angular documentation set. This topic
attempts to provide the basics of content projection for developers. It
is not a comprehensive guide; rather, it is a starting point that we
can hopefully expand upon in the future.
PR Close#41143
