Slide UI button to open in full screen

For the https://github.com/Polkadot-Blockchain-Academy/pba-site :

> Independent of how large your screen is. A button to view at full size would probably be good

Related to #28 , but a smaller and more pressing ask: we want a way from the _book_ interface to quickly trigger a fullscreen mode of the slides that usually uses `f` - this still works on desktop! but _not_ on mobile