Stable documentation contains broken links to nightly documentation

[jethrogb]

On the stable std::fmt::Debug page, under implementors, there's a bunch of links to the nightly documentation of the in-tree rand crate. What's more, the links don't work.

This first appeared in the 1.16 docs. Problem still exists in beta as of the 1.20 release candidate.

[kennytm]

Duplicate of #43701.

[shepmaster]

Closing as duplicate