Document compile-time attribute metadata for Validator and Serializer

[lacatoire]

Summary

• Document compile-time attribute metadata discovery for the Validator component (issue [Validator] Allow using attributes to declare compile-time constraint m… #21344, [Validator] Allow using attributes to declare compile-time constraint metadata symfony#61528)
• Document compile-time attribute metadata discovery for the Serializer component (issue [Serializer] Allow using attributes to declare compile-time serializati… #21345, [Serializer] Allow using attributes to declare compile-time serialization metadata symfony#61532)
• Document the deprecation of ClassMetadataFactoryCompiler in Symfony 7.4

Fixes #21344
Fixes #21345