Create documentation about designing and naming new Qudi icons

[drogenlied]

There are good outlines and "do/don't" lists for the Oxygen project. We can adapt these.

[liuyanulm]

Qudi icons are basically all finished. I believe this issue can be closed.

[Neverhorst]

It's not about if "we have all icons". Qudi is constantly expanded. Also by adding new icons (created some myself recently). This issue is about writing a proper how-to for adding new artwork to qudi.