Closes #1070.

This PR:

- updates `jekyll-anchor-headings` to `1.0.12`; this was a simple copy-paste
- updates `lunr.js` to `2.3.9`; this was a bit more involved:
    - I didn't see a minified build in the repo, so I ran it through [DigitalOcean's minifier](https://www.digitalocean.com/community/tools/minify)
    - copyright notices weren't properly included in the previous minified build, so I:
        - include an actual copy of the original MIT License for `lunr.js`
        - includes proper attribution for other functions, which include derivative works

There's a tiny bundle size increase in `lunr.js` due to the comments, but I think that's reasonable given that it's related to licensing; still trivial in the grand scheme of things.

As an aside: it would be neat if we could include minification as part of the build pipeline instead!  

Fixes a bug where content after headings in header elements disappeared.
Header elements are not currently used in JtD, but could make the HTML more semantic.

I ran Lighthouse on just-the-docs and noticed the anchor links
were missing a "discernible name".

This adds aria-labelledby to the header element to improve these for
screenreaders.