Write docs for particle packing #529

[LasNikas]

[efaulhaber]

Packing docs exist now, but if you are actually looking to pack something, they're not really helpful. I would expect an example somewhere. Maybe just add a section "Example" and refer to the relevant example file? Or maybe make this a Literate.jl tutorial on packing?

[LasNikas]

Good idea. See #847