Merge: gamnit: add documentation and standardize the API

Improve the documentation for the central services of gamnit, mainly for entities visible from gamnit.org. Also, to improve the API, remove the `Gamnit` from some class name and move a few named constuctors to factories or their own class.

Pull-Request: #2422
Reviewed-by: Romain Chanoir <romain.chanoir@viacesi.fr>
Reviewed-by: Jean Privat <jean@pryen.org>

diff --cc contrib/model_viewer/src/model_viewer.nit
Simple merge

diff --cc lib/gamnit/flat.nit
Simple merge