Add convention on asset retention

[lmgeorge]

Why While authoring a different feature, it became apparent that we do not store multiple variants of asset "just in case". We can re-export from our authoring tools (i.e., Figma) if need be.

This adds a note why we do not do this for developers from the future.

[skip changelog]

[lmgeorge]

@aduth - here's that tiny PR I said I'd make on our how we author and version control assets.