Update generics reference to use PEP 695 type parameter syntax

[BHUVANSH855]

This PR updates the reference/generics.rst documentation to reflect the modern type parameter syntax introduced in PEP 695.

Changes include:

• Updating initial examples (e.g., Stack, Box) to use class Foo[T] syntax
• Adding a note explaining the transition from TypeVar/Generic to PEP 695
• Retaining mention of older syntax for backward compatibility

This improves consistency with the spec documentation and reduces confusion for users of Python 3.12+.

Fixes #2227