Documentation: missing references in Guides, https://hyper.rs/guides/client/json/

There's a lot of missing references on the guide page.  It'd be a  nice little improvement to have the highlighted text parts be resolvable  links. 

This can be considered an easy fix but it improves the value of the documentation significantly.

One example of this would be the referenced `example` in https://hyper.rs/guides/client/json/ links to https://docs.rs/hyper/*/guides/client/basic/


