Add `cargo` sh-completions to install guide

### Problem you are trying to solve

Improve UX/DX for users who want full shell-completions

### Solution you'd like

[/doc/user-guide/src/installation](https://github.com/rust-lang/rustup/blob/master/doc/user-guide/src/installation/index.md) should include shell-completions (at `#enable-tab-completion-for-bash-fish-zsh-or-powershell`) for `cargo` too.

### Notes

_No response_