docs: publish as ebook/plain html/pdf or other formats

As discussed on discord, would the team please consider generating ebook versions of the documentation as an additional artifact of the site build?

I've tested manual conversion with `pandoc` which looks promising, but obviously the output needs tweaking and the process is not nice. e.g. I had to manually parse sidebar.json to get the correct ordering of the articles.

Whereas this might get closer to the right thing:
https://www.gatsbyjs.org/packages/gatsby-plugin-ebook/#gatsby-plugin-ebook

thanks all. stay safe-

UPDATE: Jump to https://github.com/iterative/gatsby-theme-iterative/issues/35