Add beginner-friendly summaries for model package classes

[Swargam-Udayasri]

This pull request adds a new Markdown file named model-summaries.md that contains beginner-friendly explanations for the main classes in the model package:

• BaseEntity.java: Describes the base class for entities with an auto-generated ID and a method to check if the entity is new.
• NamedEntity.java: Adds a name field to BaseEntity, used for entities like PetType and Specialty.
• Person.java: Represents a person with first and last name fields, extended by classes like Owner and Vet.
• package-info.java: Explains the use of @NullMarked for null-safety across the package.

These summaries are written to help new contributors understand the structure and purpose of these core domain classes.

Closes #2