Add the new py:deco cross-reference role

[AA-Turner]

Purpose

Introduces a new cross-reference role for Python decorators that prepends an at-sign (@) to the referenced text.

References

• Resolves Add support for a Python decorator role #13105
• Docs: make it easier to reference and markup decorators python/cpython#105812
• Improve docs for typing.dataclass_transform python/cpython#105792 (comment)
• gh-105812: [PoC] add :deco: role and adapt some rst files python/cpython#118743