Document API #946

See easystats/easystats#73

To add support for models, especially for parameters and including all features, we need following methods:

• insight::find_formula()
• insight::model_info()
• insight::get_parameters()
• insight::get_statistic()
• insight::find_statistic()
• insight::get_varcov()
• insight::get_df()

In other packages, these are optional, when the above methods work fine:

• parameters::ci()
• parameters::p_value()
• parameters::standard_error() or insight::get_varcov()

Nice to have (though some could already work if the above methods are implemented):

• insight::find_parameters()
• insight::get_modelmatrix()
• insight::get_predicted()
• insight::link_inverse() and insight::link_function()
• insight::get_residuals()

The simple way is just adding a method for parameters::model_parameters(), however, than this model-class is not supported by all functions we offer in the easystats eco-system.

I think we need a vignette on the API, explaining how to add methods/support for new models, similar to what @mattansb has done here: https://easystats.github.io/effectsize/articles/effectsize_API.html

Maybe we can split those vignettes into two (or more?) parts:

• What to do to add new methods for extracting/tidying parameters? That would only require a method for model_parameters().
• What to do to add new methods for some easystats support? (I'm thinking of check_model(), which seems to be often used by users) This would probably require some more methods - must check.
• What to do to add new methods for full easystats support? This would need methods in insight, maybe also parameters or bayestestR.