Some small improvements 🔧

We decided to make some small changes for better playability of the prototype:

- The "how to play" box in the stage selection now has a more precise and complete description
- Up and down movement has been eliminated; it was superfluous
- Increased braking friction factor for more precise movement control
- The floppy disk now rotates on itself, it seemed too static before
- Added an initial warning message for those who are photosensitive. We care about your health