ENH: Clarify HPI coil order in plot_alignment

[larsoner]

During acquisition, MEGIN systems detect which HPI coils are which. In plot_alignment(..., dig=True) we could:

1. Add hpi_colors="auto" where it will make the HPI dig colors match those of the MEGIN HPI cables when using MEGIN data, and say what those colors are (but also make them user-configurable)
2. Add a text overlay over the HPI coils that tell you which digitized coil it was

I think this would give people the info needed to know what the detected dig-to-coil order is.

Inpsired by #13484