[Docs]: Enhance the documentation on the usage of Ext/NamedExt types

[urlyy]

Description of the current issue:

We need to remind developers :

1. After defining and register a custom serializer for a type, the type_mapping of that type corresponds to the Ext or NamedExt type in Fory.

We need to remind users:

1. When using a type with a custom serializer, both peers must perform the same registration operation; otherwise, Fory will not know how to deserialize it. Moreover, the serializer logic must also be identical, otherwise deserialization errors may occur.

Link to the page, allows developers to better identify problems:

Maybe: https://fory.apache.org/docs/docs/guide/xlang_object_graph_guide

Would you like to fix this issue?

None