New example equivalency for spectroscopic conversion

I've added a new example to the docs for spectroscopic unit conversion, specifically for velocity conversion, including an appropriate reference for the various velocity conversion conventions.

One question this raises, though: why don't equivalency conversions return the requested unit?  I would expect `u.Hz.to(u.angstrom,1e15,equivalencies=u.spectral())` to return `<Quantity 2997.92458 Angstrom>` instead of `2997.9245799999994` as it does.