[DOC] Document the custom model config files

[clefourrier]

[ParagEkbote]

Could you please point to the files/ folder you'd like to document? I'd love to open a PR to fix this.

cc: @clefourrier

[clefourrier]

I don't think we have a lot of documentation on our model creation files (the yaml files you can use to load models) (though it's been improved since the creation of this issue) - you should look for the ModelConfig example files (here) and all the Config classes at the top of each model file in this folder. It would make sense to open 2 PRs: one to update the docstrings of all config files, and one to add a "Create a model" guide page in the doc :)