Add `.repeat` method to 'Animations tutorial' page

[feinstein]

Page URL: https://docs.flutter.dev/ui/animations/tutorial
Page source: https://github.com/flutter/website/tree/master/src/docs/development/ui/animations/tutorial.md

Description of issue:

On "Monitoring the progress of the animation" the example shows how to use forward and reverse, but I believe it should also have a note that there's repeat.

[sfshaza2]

Good point!