Document very low quality voices from Apple

[HadrienGardeur]

Apple devices come preloaded with Eloquence voices. While they're not as harmful as the novelty voices, they're clearly a few steps behind what's otherwise available on these platforms in terms of quality.

I'll document these voices in a separate JSON file in order to allow implementers to easily filter them out from their voice selector component.

I'll also open a separate discussion about voice quality and whether we'd like to convey that information in these JSON configuration files.