Add authors/contributors to guides

Display the avatars and names of users that have contributed to a guide, either at the top or bottom.

For the time being, let's just hardcode this inside of the guides. Someday we'll rewrite web anyways, no point in writing a fancy system for this. (Although a custom tag like `<author avatar="" name=""></author>` is fine, can be handled by the docs renderer and is fairly easy to code in.)