Add @docs_group decorator to generated models

[vdusek]

Context

In PR #638, we added @docs_group decorators to all public classes for improved API reference documentation grouping. However, src/apify_client/_models.py is a generated file, so the decorators were reverted.

Task

Add @docs_group('Models') decorators to all model classes in _models.py. Since this file is generated, this needs to be done via post-processing (e.g. a script that adds the decorators after code generation) rather than editing the file directly.

This includes:

• Adding the from apify_client._docs import docs_group import
• Adding @docs_group('Models') to all model classes (BaseModel subclasses, StrEnum subclasses, etc.)
• Integrating the post-processing step into the code generation pipeline