Improve weakref_slot docs in dataclasses.rst

[sobolevn]

While merging #123342 I've noticed that we can improve weakref_slot docs. Here's how it looks now:

I think that we can add a link to weakref.ref under "weakref-able" term. Like this:

Right now dataclass.rst does not have a single link to weakref.rst. This change will make it easier for users to navigate to this term, which not all people are familiar with.

Linked PRs

• gh-123570: Add link to weakref.ref from weakref_slot docs in dataclasses #123571
• [3.13] gh-123570: Add link to weakref.ref from weakref_slot docs in dataclasses (GH-123571) #123594
• [3.12] gh-123570: Add link to weakref.ref from weakref_slot docs in dataclasses (GH-123571) #123595