docs(aio): add ngATL header

[StephenFluin]

Fix #20568

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit

PR Type

What kind of change does this PR introduce?

[X] Documentation content changes

[mary-poppins]

You can preview 6756eb5 at https://pr20590-6756eb5.ngbuilds.io/.

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}