Create user guide for how to query collections #207

This guide will address one of our top FAQ, and should explain how easy it is to query for specific sets of posts, including

• How to filter collections, e.g. by search term, by ?slug, or by ?tags= or ?categories=
• How to limit collections by date with before and after
• How to retrieve posts matching a category by slug, by requesting the category by slug first and then making a request for posts w/ that category's ID