docs: Document the tail field option for inherited models

[marcelomendoncasoares]

Summary

Model fields can now be marked with tail so they are ordered after every normal field of an inheritance hierarchy. This is what keeps audit fields like createdAt and updatedAt at the end of a generated class instead of in front of the child's own fields. Neither the inheritance page nor the model reference mentioned it.

Changes

• Add an "Ordering inherited fields" section to the inheritance and polymorphism page.
• Show the field-level tail syntax and how it combines with other field options.
• Walk through a three-level hierarchy and list the resulting field order.
• State that child tail fields come before parent tail fields, that the order applies to constructors, serialization, and table definitions, and that tail cannot be used on id.
• Clarify that tail affects generated code only and does not produce a column-reordering migration.
• Add the tail (fields) row to the model reference table.

Reference

• feat: Add the tail keyword to configure inherited fields order on models serverpod#5394
• Add posility to change position of database columns which are inherited from parent model serverpod#5355