Docs for field-guide

field-guide needs its own docs page that outlines how to use it. I'm sure we all know that already, just figured a tracking issue would be nice for launching that effort. :)

The issue names no files, tests, or entry points. Start by reviewing the field-guide repository's current documentation structure and conventions; done means adding a dedicated page that explains how to use field-guide.

Written by the indexing model from the issue text.