[Feature] implement search functionality

[mansona]

As we are going to be encouraging people to migrate from ember-cli-addon-docs (ECAD) to Field Guide we need to consider implementing some of the core functionality that ECAD provides.

ECAD provides a search implementation using https://lunrjs.com/ that builds an index at build time and has a search interface in the app. We should consider implementing something similar so that people can migrate without losing this functionality