Added How to Play screen

During a recent playtest, some of the testers weren't clear on the movement and combat mechanics.  People also didn't quite understand what the enemies were until they spoke.

To address this, I added a "How to Play" screen that explains the movement and how the combat works, and added a bestiary of the enemies.

I created prefabs for the item info blocks and allowed them to be configured individually with the title, description, and image.  I also used the handy OnValidate() method to display the correct information in edit mode.