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)
I was thinking that we should have an example just about the "intermediate" `Json` enum, mention its variants, and how to create them by hand. And after that, have this example which is mainly about `from_str`