Make sprites transparent

[zeljkot]

Current version looks very ugly in C4 diagrams:

In comparision, logos look much better:

[Potherca]

It looks like this is caused by the original images not having a transparent background.

For instance has a white background.

The new images, for instance do have a transparent background.

This might be a good time to update the source images to the new location at https://github.com/kubernetes/community/tree/master/icons and re-generate all the sprites.

That would also resolve #3.

[Potherca]

Hmmm. Might be related to plantuml/plantuml#537 (which was not available/know at the time this project was created), will need to check.