Add support & info about v6, v7, and v8 UUIDs

[tgross35]

There are three new UUID variants: v6, v7, and v8. They aren't IETF official yet, but that is expected for 2023 I believe (and use is already widespread). It would be great to mention them on the "explained" page, and a diagram like you have for V1 UUIDs would be awesome for V6 (and honestly all other types)

Link to the relevant spec: https://github.com/uuid6/uuid6-ietf-draft#high-level-overview and the rendered draft IETF doc https://www.ietf.org/archive/id/draft-ietf-uuidrev-rfc4122bis-00.html