Document the Aspect, State, and Properties system

[mxgrey]

There's some crazy complicated template black magic in dartsim. Most of it is hidden deeply away inside the implementation, but it may be discouraging to newcomers who are interested in contributing to the codebase.

I should document the design of the implementation. Hopefully that would make our implementations more approachable to new developers.

[psigen]

FWIW for anyone wanting to track this issue, I think these are the major relevant PRs, including some very good explanations @mxgrey previously generated:

• ShapeFrame and ShapeNode (initial inspiration for Aspects): VisualNode & CollisionNode #394 -> ShapeFrame and ShapeNode #608
• Nodes and Addons sketch: [Preliminary] Nodes and Addons #531 -> Revision of #531 #598
• Optionality of Addons: Changing authority of Optional/Required Addons #607
• Refactor to current Aspects, States, and Properties: Aspects, States, and Properties #659

Wow, we hammered on this for a while, huh 😅