Include user-facing info about scale bars

## 📚 Documentation
As far as I've found, there isn't any user facing information regarding showing the scale bar and scale bar units. There is an example (and it's very good! https://napari.org/dev/gallery/scale_bar.html?highlight=scale%20bar ), but I think this is such a fundamental thing that it should be covered in a higher level user-facing set of docs.
Probably best for the viewer tutorial.