Update theme and docs site to use latest Hugo technology

[bep]

This is certainly related to #1291 -- but I don't think it's time to do a full revamp of the theme right now.

But there is a saying that you should Eat your own dog food -- and we're certainly way behind the Hugo train in the docs site/theme, making it harder to maintain than it should be.

So, I suggest that we do a refresh with these short goals:

• Upgrade the CSS build setup to use TailwindCSS 2 and Hugo's build system (see https://github.com/bep/hugo-starter-tailwind-basic for a technical setup)
• Upgrade the JS setup to use js.Build

The above means that:

• We use NPM/package.json only to pull in dev dev-dependencies for TailwindCSS, JS -- all building happens in Hugo

We already use a very old version of Tailwind. I suspect there are some scripts "out there on the internet" that can help with the replacement of the "old => new" utility CSS class names.

UPDATE: I notice that the current version uses Tachyon for CSS, but the above still applies in general.

[ytrepidorosonomous]

Already in progress here. Correct?