Build main.css using npm build

[alanorth]

This bumps the version of Bootstrap from 4.3.1 to 4.4.1 and introduces a build system using npm to compile and minify the Bootstrap and local style overrides into one CSS file (main.css). If you have npm installed via your system's package manager (like apt on Linux or Homebrew on macOS) you can re-build the CSS like this:

$ npm install
$ npm run build

I want to start improving the documentation by making it less hand-curated and more programmatically built, syntax checked, etc. The idea here is that we build the documentation in a deterministic, repeatable and reliable manner, without relying on any third-party servers being available and without pulling in dependencies that might have changed.

[marieALaporte]

You can merge this one too