Document game import/export package structure

[evktalo]

The package structure the editor uses for loading and saving projects needs to be defined.

It might be wise to keep the structure closely aligned with how the editor does it so that deploying and running the created games is as simple as possible.

Current package structure:

pelidata.zip
├── images
│ └── placeholder
│ ├── furniture.png
│ ├── item.png
│ └── room.png
├── interactions.json
├── items.json
├── rooms.json
└── texts.json