Slide UI button to open in full screen

[nuke-web3]

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