Merge: Document Nit Serialization

The Nit serialization system did not have any documentation. This is the base to a better documentation.

In a next PR I may make more services of the serializers private. Implementations, such as json_serialization, will need to intrude import serialization. However, the end-user will only see the basic services.

Pull-Request: #1202
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Philippe Pépos Petitclerc <>
Reviewed-by: Frédéric Vachon <fredvac@gmail.com>