Add Fig as an autocomplete method

[brendanfalk]

Hey there! Fig provides autocomplete for 300+ CLI tools including rustup. It looks like this:

We think this will make users more efficient with rustup CLI, so we would love to have Fig listed as an autocomplete method in your docs. Thanks so much and please let me know if you have any questions!

[kinnison]

The way you amended the docs here suggests that we (Rustup) can affect Fig's autocompletions because we're saying the support is there - is that true? is fig using our completions data or is it somehow built into Fig without reference to the Rustup itself?