Symbolic Icon Howto

Use [Symbolic Preview](https://flathub.org/apps/details/org.gnome.design.SymbolicPreview) to render the icons. [Read more about the process](https://blog.jimmac.eu/2021/how-to-symbolic/) and [specs](https://gitlab.gnome.org/Teams/Design/icon-development-kit/-/blob/main/README.md).

