[Request] Add a search plugin

[Daenyth]

One thing that I think would make the scala documentation situation globally improved is for the common doc tools to provide search over the doc pages.

The mkdocs tool for example does this, and it makes discovery a lot better. It uses https://lunrjs.com/ to implement, but I'm sure there's other tools if that's hard to integrate for some reason.

[sloshy]

Progress update: after taking some time to get the build working again, I've got working search index generation. I'm playing around with the generated html/css layouts to see if I can make it look half-decent before I throw it up for review, plus I still have some JS to write/integrate, but, so far so good!