Generate Preview Images

Similar to how GitHub generates preview images when you link a repository on GitHub, it might be interesting to have a way for Zola to do the same.

I understand this is probably a pretty niche request, so instead of Zola generating an image, maybe we could add an external_build option to configuration.toml that runs a shell command every time zola tries to build? A little hacky and it would allow for arbitrary complexity, but sometimes I need arbitrary complexity :sob:

(example of a GitHub preview image below…)