Add gh-pages based docs

README.md grew large so it would be useful to structure the docs in a gh-pages based site.
We can use: [jekyll](https://jekyllrb.com/), [mkdocs](https://www.mkdocs.org/) or [hugo](https://gohugo.io/documentation/) as doc site generator

Start with README.md, then compare the proposed Jekyll, MkDocs, and Hugo options for structuring the existing documentation as a gh-pages-based site. Done means the documentation has a selected site generator, is organized beyond the single README, and is available through gh-pages.

Written by the indexing model from the issue text.