Add Documentation for #[IsSignatureValid] Attribute

This PR introduces documentation for the new #[IsSignatureValid] attribute, which enables declarative request signature validation at the controller or action level.

✅ Included in this PR:

• Basic usage of #[IsSignatureValid] with default behavior
• Support for custom failure status code via validationFailedStatusCode
• Option to throw exceptions using throw: true
• Method restriction using the methods option (supports both string and array)
• Example of applying the attribute at the controller class level to protect all actions

Code PR

• [HttpKernel] Add #[IsSignatureValid] attribute symfony#60395