[RF DOCS] Active Model Basics Documentation [ci-skip] (rails#51210)

Review the Active Model Basics guide.

Add `Attributes` and `AttributeAssignment` sections.

Update the rest of the sections by trying to provide more information, be more clear, and include more examples. In some parts, it references the API where more information about the methods can be provided.

Expand `ActiveModel::API` and `ActiveModel::Model`, reorganize sections to a more sensible order.

Co-authored-by: Carlos Antonio da Silva <[email protected]>

Author: Ridhwana