Auto merge of #6654 - ehuss:alt-stabilize, r=alexcrichton

Auto merge of #6654 - ehuss:alt-stabilize, r=alexcrichton

Stabilize Alternative Registries

This includes a few minor changes besides stabilizing:
- `cargo search` won't display `crates.io` search URL for extra results for non-crates.io registries.
- Document using environment variables for API tokens.
- Explicit section in `config.md` for documenting the credentials file.

Closes rust-lang/rust#44931
Closes rust-lang/crates-io-cargo-teams#21
Closes #6589
Closes #4688