hide slice::{Concat, Join} from docs

[cyrgani]

Per #27747 (comment), these two traits are implementation details and will not be stabilized, so let's hide them from the docs.

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[hanna-kruppe]

The traits will never be stable, but their definition and impls are still important for understanding the signatures (possible element types and corresponding input types) of the stable concat/join methods.

[petertodd]

@hanna-kruppe I didn't actually know about join or concat before, and read the docs for the first time right now. They were a lot easier to understand when I went to the Join/Concat definitions and could see right there what impls there were and what exactly the output type was.

[cyrgani]

Let's keep them visible then.