Document new type parameter syntax (PEP 695)

[JukkaL]

#15238 is about overall support for PEP 695.

[TeamSpen210]

This might be overcomplicating things, but Cython has a similar need to display two code styles, which they handle via the sphinx-tabs extension. That allows only showing the variant you want to use, making the page a little easier to read. If these are sticking around for a while in docs, maybe Mypy should do the same?

[JukkaL]

An interesting idea. I don't have a strong opinion -- we'd need to experiment with it before making a decision. I'm not going to switch to tabs at the moment, but we can consider it in the future.

[cdce8p]

This might be overcomplicating things, but Cython has a similar need to display two code styles, which they handle via the sphinx-tabs extension. That allows only showing the variant you want to use, making the page a little easier to read. If these are sticking around for a while in docs, maybe Mypy should do the same?

Opened #18264 so we can decide if it's helpful.