[fastpages] Image (path) handling

Right now, the only working combination to use the same image in metadata and content is having it in images/ and using an absolute (github user content) link within the notebook. That is sub-optimal, especially if the image is not yet committed at all, or changed locally. Note that “working” includes locally (JupyterHub using the git workdir), during build, and on gh-pages.

A simple thing would make this a lot better: copy an existing symlink _notebooks/images → ../images verbatim to the final gh-pages, then all paths are image/… whereever they appear. Right now, that symlink is read somewhere and put into the image paths, breaking them.

Happy to take a PR on this, I believe you would have to make a PR to nbdev to fix this