Add text about `model` inside `model` not supported

[glihm]

Models inside models is not supported due to how keys are retrieved. We should add a warning about this in the model section.

The workaround is to have a struct with Introspect derived instead. We could add also a reference to that into the introspection page: https://book.dojoengine.org/framework/models/introspect.

[Simplyauf]

Could I grab this task?

[codeAKstan]

Dojo does not support nested models (i.e., defining a model inside another model). Each model must be declared at the top level. To establish relationships between models, use references or define each model separately.