[feature][legend] Make Legend Items Clickable + Update Docs

[hydrosquall]

Summary

• Solution for #535 by adding an optional customClickBehavior property to the Legend component API.
• Renames some internal code to make it clear that either the code for horizontal OR vertical related code is called, but not both.

Example

• GIF of click behavior working on the old docs page: https://a.cl.ly/o0um9KdA

Discussion / Future Proposals

• customClickBehavior is a bit verbose, but I chose it to match the pattern used by the top level Frame and InteractionLayer APIs. I felt it made more sense to put this prop on the whole legend rather than providing a custom customClickBehavior for each legend group (as I originally commented in the linked issue), but can revise this if needed later.
• A future refactor could make the shared behavior between the vertical/horizontal renderers a bit more explicit
• We may want to port these legend docs over to the semiotic-docs repo
• We may want to document the existence of the Legend property on components like the NetworkFrame, otherwise it's unlikely people will discover this component

[bengarvey]

I think this will work for what I was going for. Thanks!

[bengarvey]

@emeeks any chance this make it into a release?