Provide icons in myst #43
Description
Right now, the scienceicons extension adds a role {science} for icons, but those cannot yet be used to render icons.
In jupyter-book/mystmd#1361, @agoose77 suggests a more general route forward with an icon role.
This would be incredibly useful, since icons are required in footers, headers, and all sorts of places.
See, e.g., jupyter-book/myst-theme#565, in which we need the icons for a footer feature.