Docs: Feedback for Implementing Java Serializable and Externalizable

Hi, I have some feedback about [this page](https://github.com/hazelcast/hz-docs/edit/main/docs/modules/serialization/pages/implementing-java-serializable.adoc)

I have following simple class

import java.io.Serializable;
public class Test implements Serializable {
private static final long serialVersionUID= -23482983298L;

private String key;

private String value;

public Test(){}

public Test(String key){this.key=key;}
//getter setters
}

And I am getting HazelcastSerializationException: There is no suitable serailizer for class Test

Start with docs/modules/serialization/pages/implementing-java-serializable.adoc and compare its guidance with the reported Test class and HazelcastSerializationException. Confirm what the page needs to explain, then update the documentation so the expected behavior and requirements are clear for this example.

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