Allow versioned images / resources inside the spec folder

Hello. Would it be possible to allow to add versioned images inside the spec folder? Through several round of trial and error I found out that this is not possible.

My preferred option is to have a `spec/v1.0.0/images` folder (inside the spec folder directly). But it is currently not possible to tell spec-up to copy the images in that folder to the root.

I had created this images folder in my spec: https://github.com/decentralized-identity/credential-schemas/tree/main/spec/v1.0.0/images
But it seems like the images folder does not get published to the gh-pages branch: https://github.com/decentralized-identity/credential-schemas/tree/gh-pages/v1.0.0

For now I have opted to create a `/images/v1.0.0` folder at the root level instead, This is similar to what I believe Sam Curren did for the didcomm spec: https://github.com/decentralized-identity/didcomm-messaging/tree/gh-pages/docs/collateral-v2.0-snapshot

Also it is fine if you decide to make the official guidance that images / resources should be hosted in the root only, I would just suggest that we add some guidance about this in the spec-up documentation.