json: add sub-section on default de/enconding (staging area)

Cover deconding from &str, default decoding and encoding into String (ref. #86)

(Similar to the previous PR, with added encoding)
Before this, we should have an example about `Decoder`. Basically `Json` -> `Decoder` -> `T`. Where `T` can be `String`, `Vec` or `HashMap`. That way, this example can focus on the `Decodable` trait