Update promotional material for version 2.0 release

[Fydar]

Unity's ScriptableObject icon has updated, and it's about time I stopped using it to promote the asset.

It might be a good idea to update the promotional material with a new icon.

Collating all of the screenshots and graphics on this GitHub repo would allow for a lot greater organisation than I currently have.

• [x] Create a recognisable icon
• [x] Create a store-page screenshot template
• [x] Create key art at multiple resolutions and sizes