Port docs from derby-site repo to this repo, with Jekyll on GH Pages hosting

[ericyhwang]

The current Derby docs in the derby-site repo run as a custom Node server manually built and deployed with Docker. This makes doc updates a pain to do.

This PR ports the docs to a Jekyll-generated site, using the Just the Docs theme, built and deployed with GitHub Actions to GitHub Pages hosting.

In addition to being easier to maintain, the new docs also automatically support full-text doc search with autocompletion.

Notes:

• I'd tried using the simple built-in GH Pages integration, but it's on the older Jekyll 2, which has always-on Liquid template processing that errors on the {{}} mustache-style syntax in the Derby code examples. Jekyll 3 has a config option to disable Liquid, but it requires a GitHub Action to do the build and deploy. Thankfully, there was a starter workflow to base it off of.
• For now, the content's copied over from derby-site, with addition of Jekyll metadata in each Markdown file. I haven't done any editing, thorough checking for broken links, or theme customization. That can come in follow-ups once we have the basic setup working.

[craigbeck]

👍