You can totally have a cut scene, either before or after the game play (or both, who am I to limit you ;-). Similarly, menus, settings, and so on don’t count towards the 20 second game play limit.
The jam needs you to have a web-playable version of your game hosted on itch.io. According to our FAQ, you can use https://packager.turbowarp.org to create an HTML version of your Scratch project
be sure to add a Skip to any cutscene :) i love cutscenes but once you view it a couple/few times, i'm ready to get back to the game ASAP :)
meanwhile, when i saw this Q posted, i thought of this 20s Masterpiece which contains great cutscenes