Portable, offline build of documentation website

[msgilligan]

We should have a portable, reproducible, GitHub-independent build of the documentation website.

My proposal is to use Nix/Nixpkgs to do this, but if you have other ideas please comment.

[msgilligan]

My proposal is to crate a Nix flake that has both a devShell and a Nix (sandbox) package build for the website.

This will allow contributors to develop on macOS, Linux, and Windows (running Nix under WSL, hopefully) using the devShell. Will allow reproducible (at some level) builds of the site using the package build. We can also perform builds under GitHub actions while still testing/validating a GitHub-independent build process.

PR #61 is a proof-of-concept that works pretty well -- see the documentation in the README file.

Once we agree on an approach and test the built-site a little, we can replace the default GitHub site publishing with an added GitHub Actions job in jekyll-build.html that published the Nix/Jekyll-built site to the GitHub pages infrastructure.