docs: Document dynamic model fields

[marcelomendoncasoares]

Summary

Models, endpoint parameters, and return values support dynamic, but nothing documented it. This adds a page for it and wires it into the places a reader would look first.

Changes

• Add a "Dynamic fields" page under Models, covering when to reach for dynamic over a typed field, the model file syntax, nullability, the supported value types, endpoint use, JSON and JSONB persistence, copyWith behavior, and the tradeoffs.
• State that Serverpod carries runtime type metadata in the serialized value, and that the encoded form is an implementation detail.
• Cover project, module, and shared package model boundaries.
• Renumber the pages after it in the Models section.
• Link dynamic from the model type list, the endpoint parameter types, the database data representation section, and the model reference.

Reference

• feat: Add support for dynamic fields on models and endpoints serverpod#5013
• Asymmetric handling of Map<String, dynamic>: serialization works, deserialization throws serverpod#4996, Map<String, dynamic> as an Endpoint Parameter serverpod#3231
• Beta fixes: fix: Fix copyWith method on dynamic fields serverpod#5146, fix: Fix dynamic data being serialized as an encoded string serverpod#5153, fix: Fix serialization/deserialization of project models on dynamic fields of module models serverpod#5157, fix: Fix the serialization of dynamic classes when importing both client and server serverpod#5201

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