Add icon for GDScript track

[pfertyk]

All initial exercises are implemented and the GDScript track will be ready soon ;) We're adding the docs etc. in preparation for the launch. We also need a track icon.

GDScript does not have an official logo. This language is part of Godot Engine, with Godot Editor being the default IDE.

Together with @ErikSchierboom and @BNAndras we decided to use this icon (an official logo of Godot Engine):

https://godotengine.org/assets/press/icon_color.svg

The license for all Godot Engine's icons is Creative Commons Attribution 4.0 International.

The official Press Kit can be helpful here as well.

[ErikSchierboom]

@iHiD Something for you