Add `Custom Object Serialization` into docs

[p-rial]

Please add how to do custom object serialization in order to be set as value in DictDB.

Example:

import score.DictDB;

public class Example {

  class Custom {
      ...
  }

  private final DictDB<String, CustomObject> temp;
  
  ...
}

Some resources and example I got from the discord channel

https://github.com/balancednetwork/balanced-java-contracts/blob/release/bBaln-core/token-contracts/bBaln/src/main/java/network/balanced/score/tokens/db/LockedBalance.java

https://docs.google.com/presentation/d/1S24vCTcPJ5GOGfPu1sApJLwyOTTdgYEf/export/pdf

https://www.javadoc.io/doc/foundation.icon/javaee-api/latest/score/ObjectReader.html
https://www.javadoc.io/doc/foundation.icon/javaee-api/latest/score/ObjectWriter.html