[FEATURE] improve API documentation

[mgovers]

Describe the feature request

The API exposes some functionality in modules via the __init__.py. E.g. a user may call to power_grid_model_io.functions.has_value.

However, the documentation refers to several API functions by their private module, e.g. power_grid_model_io.functions._functions.has_value(value: Any) → bool

This feature is about updating the documentation so that it refers to the public module that exposes the function, rather than the private module in which it is defined.