docs(serialization): decorate the denormalizer to change how a payload is read

[Amoifr]

Refs api-platform/core#8550, follow-up to api-platform/core#8606.

Since 4.4, payloads are denormalized by dedicated *.denormalizer.item services. #8606 keeps a normalizer decorator that implements DenormalizerInterface working, with a deprecation that asks to decorate the denormalizer instead, but the serialization page still shows that deprecated setup as the way to go. This updates it:

• ApiNormalizer now only normalizes, with the symfony/serializer 7.4/8 signatures (the previous example was untyped and would not load anymore);
• a new "Changing the Denormalization" part shows how to decorate api_platform.jsonld.denormalizer.item, links to the upgrade guide for the other formats, and says the old setup still works until 6.0 with a deprecation.

Both classes were run against symfony/serializer 8.1.7.