docs: Add Experimental Rust Guide (#13375)
## Why
Turborepo now has native experimental Cargo workspace support, including
standalone Cargo repositories, so it needs dedicated guidance. The
generic multi-language guide should continue to describe
unsupported-language integration.
## What
Adds an experimental Rust tool guide and links it from the Tools index.
Reworks the multi-language guide around a Go service using package
scripts and package boundaries.
## How
Documents Cargo-only and mixed JavaScript/Cargo repositories, inferred
Cargo tasks and their configuration, caching, filtering, queries, and
current limitations. Uses a Cargo-only workspace for the minimal Rust
example.
