Add tiny example app with Product/Category, and end-to-end tests

[mustafaguler3]

This PR adds a tiny example Spring Boot application demonstrating the JSON:API library.

• Added Product and Category models with JSON:API annotations
• Created ProductController with endpoints
• Added end-to-end tests using MockMvc to verify JSON:API responses
• No additional controllers needed, Category is included via relationships

[MieskeB]

I will have a look at it tonight!

[mustafaguler3]

I will have a look at it tonight!
Thanks for taking a look!
if there is anything i missed or that could be improved, i'm happy to update the PR.