Draft of docs on creation of backend and model galleries

[rampa3]

@mudler I would like to present my drafts of possible documentation on how to write the YAML files for model and backend galleries.

I submit these here, as I feel this is not matter fit for an issue at first draft state, and I don't know how to properly add pages into the documentation at the moment.

custom-backend-gallery.md
custom-model-gallery.md