docs: Corrections to Custom serialization page

[developerjamiu]

Summary

• Fixed toJson example: bare name: → 'name': (wouldn't compile as-is).
• copyWith() moved out of required rules. It's only needed when the class is used as a field inside a generated .spy.yaml model (verified against test fixtures). Added dedicated section.
• Recast "three simple rules" — the list had four items.
• Added missing return type to second toJson example.
• ### Implementation Example → ### Implementation example.

Test plan

• Render locally and confirm the new section reads correctly.