Add a dedicated icon for a main branch docs site (#234)

This adds a PNG icon for denoting the development docs made from bevy's `main` branch. Added as suggested in bevyengine/bevy#3535.

If it doesn't show up in the diff, this is how it looks right now:

![image](https://user-images.githubusercontent.com/3137680/147969010-2092c921-fb53-4549-981b-8a7467cf3de0.png)

Co-authored-by: Aevyrie Roessler <aevyrie@gmail.com>