Document new search module and features

[daveaglick]

See https://github.com/statiqdev/Statiq.Framework/issues/192

[Hardcoreprawn]

Hi @daveaglick ,

I'm building a document repo with statiq and trying to get search working using Lunr. I'd been keen to see how to enable it. I can see the search.js and the index files, but I'm totally failing to figure out how to turn it on. I'll go and look through the source for the statiq.dev site to see how you've done it and if I can work with it.

Thanks for your work on Statiq, I'm enjoying it and getting good feedback on the site so far.