Docs: Add class diagram figure

[brendanreardon]

The classdiagram refers in the VRS docs to a figure showing maturity levels for individual classes, I am not sure if y'all have something similar?

Originally posted by @jsstevenson in #182 (comment)

Daniel created a class diagram figure for the cat-vrs paper draft, that we can add as an appendix once finalized.

[korikuzma]

Hey @brendanreardon ! I would recommend using Draw.io for figures in this repo. Draw.io Integration Extension in VS Code allows you to directly edit SVGs in VS Code. I started doing this in VRS (see ga4gh/vrs#679 and ga4gh/vrs#680). Personally, I would love to move towards this direction so we only have to update in one place. The image quality is also much better.