Add `staggered_pic_selection` video to 'Staggered animations' page

[iraj-jelo]

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

Description of issue: there is no demo in dartpad or a youtube video for staggered_pic_selection example in the page.